I don't deny the fact there's a possibility for malware downloaded in Linux (that won't affect anything on Ext3/4 anyway) to infect an NTFS partition, or an USB key that will in turn infect Windows if you dual-boot. But I've used Linux long enough and never saw it happen, so that's one thing. The other thing is I've been posting during years on a few Linux forums (PCLOS, Mandriva...a bit on *buntu...) and never saw anything reported about cross platform infections. First, many Linux users use just Linux, so there's no risk as 99,99% of malware needs a Windows based operating system to be effective, while many others dual-boot (like myself, though not currently), and never got a virus infection from running Linux "unprotected". So, I just won't use an AV in Linux. If Linux went mainstream (could always happen, in years, another topic...), then chances are it will be targeted, and chances are there will be appropriate software - not the ones we have now that are meant for Windows files and processes (even if "usable" but useless in Linux like Avast for Linux) - but Linux security software, new software. One last thing, I read there were about 45 acknowledged threats for Linux...not worth bothering

+ it's sounds more difficult to run an executable with elevated privilege in Linux than it is in Windows (even with UAC on and a limited account).