Windows XP
I must admit, I had nearly warmed up to this stupid OS just a few days ago. Until I set up my DSL without configuring a firewall, and got meself a nice tasty worm virus. Okay, no big deal, zap it with AVG Virus Scan, no big deal, it's gone. Configure the firewall like I should have originally done, everything's fine. Nope, there's that virus again. That's odd, I just ran a virus scan, came up empty. Whatever, quarantine the file. Fucker kept coming back and back again, but AVG wasn't picking it up. Why wasn't it picking it up!?!? Because in all their infinite wisdom, Microsoft's System Restore is designed so that no outside program can fuck with those files, and the virus was stuck in there! So about every hour or so, Windows itself would let the virus slip out, and kept looping it no matter how many times I tried to scan for it. Got rid of System Restore, virus scan, over and done with. How gay is that?
By the way, I have a dual boot setup with XP and Mandrake Linux, I just haven't gotten around to farking around with Linux all that much yet, but with shit like this happening, that day is drawing near.