A sneaky computer worm that uses a virtual Swiss army knife of attack techniques has infected millions of Microsoft Windows PCs, and appears to be spreading at a fairly rapid pace, security experts warn. Also, while infected PCs could be used for a variety of criminal purposes — from relaying spam to hosting scam Web sites — there are signs that this whole mess may be an attempt to further spread so-called “scareware,” which uses fake security alerts to frighten consumers into purchasing bogus computer security software. The worm, called “Downadup” and “Conficker” by different anti-virus companies, attacks a security hole in a networking component found in most Windows systems. According to estimates from Finnish anti-virus maker F-Secure Corp., the worm has infected between 2.4 million and 8.9 million computers during the last four days alone. If accurate, those are fairly staggering numbers for a worm that first surfaced in