We're less than a day away from a global computer meltdown — or a terrific April Fool's joke.

At some point on April 1, the Conficker virus, which has quietly infected millions of PCs worldwide to herd them into a "botnet" of linked machines, will phone home for new instructions.

What it'll do next is anyone's guess. It could muster enough silicon firepower to take down any Web site on the planet, or send out enough spam to fill the inboxes of every e-mail user on Earth.

It could offer itself up to the highest bidder, mostly likely an Eastern European cybercriminal. Or it could do what it's been doing for months — nothing.

"The biggest mystery about Conficker is 'Why?' What exactly is it that these bad guys are planning to do with it?" asked security expert Mikko Hypponen, of the Finnish company F-Secure, in a conversation with the Times of London.

• Click here to read how to protect yourself from the Conficker worm.

• Click here for Microsoft's solution.

• Click here to visit FOXNews.com's Cybersecurity Center.

• Got tech questions? Ask our experts at FoxNews.com's Tech Q&A.

Since last fall, the worm — it spreads on its own, not by human action — has spread through governmental institutions, including the British and French militaries, and corporations around the world.

But it's not clear how many machines have been infected —estimates range from 5 to 10 million.

Conficker is so sneaky, and so low-key, that the only way you can tell if you're infected is if you can't access certain Web sites, such as Microsoft's security pages or those belonging to security vendors such as F-Secure, Symantec or McAfee.

If it turns out you are infected, Microsoft's put up instructions on how to clean your PC, but it's not easy. If you're not, downloading and installing Microsoft's latest software updates should protect you.

The software giant has in fact put up a reward of $250,000 for information leading to the arrest of the virus's creators, but security experts think whoever's running it has little chance of being caught.

"No one knows who controls Conficker, so it could be controlled by terrorists," wrote tech journalist Ben Worthen in the Wall Street Journal. "It could also be controlled by the kid who played Urkel on 'Family Matters.'"

So network administrators, computer-security experts and geeks in general will be anxiously waiting to see what takes place on April Fool's Day — even if none of them will venture a prediction.

"It's a brave man who puts his neck out like that," joked Graham Cluley of the British security Sophos in a conversation with the Guardian.