major game slowdown + no task manager

recently ive been experiencing some wierd problems.
first of all, i cant open the task manager, if i press ctrl alt del, it opens briefly (1 second or less), then closes itself. it still shows the task manager icon in the lower right hand corner of the screen, but if i move my cursor over it it dissappears.
secondly, game (sof2/ut2k3/any fairly demanding game)fps' slow down suddenly, then go back up. like itll be running at 100 or whatever, then itll drop down to 20-30 for a couple/several seconds, then shoot back up, like somethings using up cpu resources... but what? and i cant open the task manager to find out! i even reinstalled windows, and it worked fine for like an hour, then it started doing it again, and i cant pinpoint the cause.

ne1 know what this is? ne1 have any suggustions?
ty

The first LAN party I went to was at a PETA convention. They booted me when I shot a crab in HL!