Never-before-seen videotapes in which Princess Diana (search) describes her sexless married life and claims her bodyguard was "bumped off" for having an affair with her are set to be aired on prime-time TV Monday night.

The revealing and private footage, filmed by Diana's voice coach during lessons to help her become a better public speaker, comes from a set of tapes found in her butler's home after her death.

NBC won't reveal how much it paid for the explosive videos after Diana's family lost its legal battle to keep them private, but the price tag is believed to be in the hundreds of thousands of dollars.

On the new tapes, Diana reveals:

* How she believes her bodyguard was fired for having an affair with her and was later murdered in a motorcycle accident.

* How she went sobbing to the queen and begged her for help after finding out her husband was having an affair with Camilla Parker Bowles (search), only to be told, "Charles is hopeless."

* How she and Prince Charles met only 13 times before their fairytale wedding.

* Her devastation during a TV interview on the day of her engagement, in which she proclaimed that she and Charles were in love and he replied, "Whatever 'in love' means."

* How Charles asked for sex only once every three weeks.

Diana recorded the tapes during 1992 and 1993 — the most turbulent years of her marriage — after hiring voice coach Peter Settelen to train her to become a better public speaker.

Diana told Settelen she wanted to establish a new public role for herself as a passionate, independent woman so she could start "making a difference" in the world.

She then agreed to his training technique — videotaped interviews in which she would talk passionately and candidly about her life to give herself confidence.

Settelen, who became a close friend of the princess, filmed her in the lounge of her apartment in London's Kensington Palace over a period of 16 months.

In the tapes, the princess is casually dressed and speaks openly about a whole range of personal topics — from her childhood, her marriage, her struggle with bulimia and her determination to win the respect of the royal family.

But the most explosive segment comes when Diana talks about her extramarital affairs and refers to the former royal protection officer, who she believes was later murdered.

Although she doesn't use his name in the tapes, it is widely believed Diana started an affair with royal security guard Sgt. Barry Mannakee in 1985. Mannakee was married with two small children at the time.

When rumors of the affair reached Charles, Mannakee was removed from duty at Kensington Palace. Less than a year later, in 1987, he was killed in a traffic accident when he was a passenger on a fellow officer's motorcycle.

In the video footage, Diana gives her own interpretation of her lover's death — which she calls "the biggest blow of my life."

"It was all found out and he was chucked out. And then he was killed. And I think he was bumped off. But, um, there we are ... we'll never know ... he was the greatest fella I've ever had."

Critics have long dismissed Diana's beliefs that Mannakee was murdered, and that her own life was also in danger, as paranoid nonsense.

Her fears of a "royal plot" have been described by her butler, Paul Burrell, in his own tell-all book.

But Mannakee's accident is being reinvestigated, along with a number of conspiracy charges surrounding the princess' own crash death in Paris in 1997, exactly 10 years later.

The new tapes also reopen wounds over the way Diana was treated by Charles and the royal family during her troubled marriage, specifically when she describes her plea to the queen for help after she discovered Charles had started seeing his former lover five years into the royal marriage.

"I went to the top lady, and I'm sobbing. And I said: 'What do I do? I'm coming to you. What do I do?' And she said: 'I don't know what you should do. Charles is hopeless.' And that was it. That was 'help.' So, I didn't go back to her again for help because I don't go back again if I don't get it the first time around."

The princess then describes how her own sex life with the prince was limited, explaining there was "never a requirement" from Charles for sex.

She also says she believes the amount of sex she had with Charles was affected by Charles' earlier relationship with then-married Camilla.

"There was never a requirement for it from him," Diana says. "Once every three weeks about, and I kept thinking it followed a pattern. He used to see his lady once every three weeks before we got married."

Charles was first introduced to 16-year-old Diana on a pheasant shoot at royal country retreat Sandringham in 1977. When the pair began formally dating less than two years later, it is widely accepted that he was already in the throes of an affair with Camilla, whom he continued to see.

In the tapes, Diana reveals how during their own brief courtship, conflicted Charles would blow hot and cold.

"He'd ring me up every day for a week and then he wouldn't speak to me for three weeks. Very odd."

While filming the princess, Settelen also asked Diana to describe her feelings during the infamous TV interview on the day of her engagement in which Charles is publicly dismissive of their royal romance to a TV audience of millions.

"I was brought up with the sense that, you know, when you got engaged to someone, you love them," says Diana. "And the most extraordinary thing is, we had this ghastly interview the day we announced our engagement. And this ridiculous ITN man said: 'Are you in love?' Oh, what a thick question. So, I said, 'Yes, of course we are' . . . and Charles turned around and said: 'Whatever "in love" means.' That threw me completely. I thought, what a strange question."

Settelen then says, "What a strange reply!" to which Diana says, "Oh, God. Absolutely traumatized me."

When the voice coach then asks Diana if she later confronted Charles about his hurtful remark, she replies, "No ... I didn't dare."

"Were you frightened?" asks Settelen.

"Must've been, yeah ... We met 13 times before we got married."

NBC is showing the newly obtained footage in two two-hour specials starting Monday after buying them from Settelen for an undisclosed sum. The network showed snippets from a different tape in March.

Settelen originally gave Diana the 20 tapes to watch in private and keep, but the set vanished after her death.

Four years ago police found six hidden in Burrell's loft, sparking a lengthy legal battle between the princess' family and Settelen over their ownership.

Settelen won that battle in September 2003, and although he pledged never to give them up out of respect for the princess' privacy, he later changed his mind, claiming he needed cash to cover his legal fees from the fight to hold onto them.

While the most explosive aspects of the tapes refer to Diana's marriage, the poignant footage also shows how the princess had a difficult relationship with her own family, especially with her stepmother, Countess Raine Spencer.

"I wanted to throttle that stepmother of mine because she brought such grief. She kept saying to me, 'Oh, but Diana, you're so unhappy in your own marriage ... you're just jealous of Daddy's and my relationship.' And jealously was not high on the agenda — it was behavior I was after."