AVI's Lock up in XP/Firefox/IE

All of a sudden…no new software loaded…I am unable to play any AVI’s. Not with Nero 8, WinAmp, WMP, etc. System locks completely up…this happens when trying to play from Website (i.e. MSNBC) or from HD or disc. XP Home SP3, 2gig meb, acres of HD space, AVG Free, Spyware Doctor. I am able to burn discs and convert discs from AVI to DVD format without a problem. Playback of AVI’s online or from PC is impossible. TIA.