well, sounds precisely like the package i had to eat lately ... same symptomes including AVsofts crashing/stopping midscan ... if its the same shit i caught, prepare to reinstall your OS
save time, check regedit for these words:
injector
total security
security 2009
UAC
and you will find a nasty bugger that blocks sites AND apps including ANY antispy/vir terms ... bundled with that, you'll find a NEW .PF file in your PREFETCH folder which reads like 145056423_Xbla.pf and 4 new .dlls and a .db recently created in system32/ ... also along with that you prolly caught NET.NET in your sytem32/ dir ... trust me, dont mess with it, it gets WORSE with every reboot and AV apps fail ... that is one seriously nasty bug