i get the same, seems to be since i installed sony vegas 5 - i think the mpeg codec has screwed up my system, as transcoding to dv/avi or wmv works fine.
dvd shrink still works fine, recode2 crashes though (which is odd as they're almost the same program).
it's not cooling, my cpu never goes above 40c, i guess it may be the psu, but that's a 500w with pretty good rails.
it's not spyware or viruses, i'm very careful about that sort of thing.
i think i'll do a fresh windows install to get rid of the bad codecs etc.
if that doesn't do it, i think i'll use this athlon for linux and buy a pentium4 for windows, they seem more stable.