Playing Video Makes the Computer Go Jittery...

There doesn't appear to be any malware in my computer... I did a few other scans and found nothing.

And yet...

If I'm doing anything video related, after about 30 minutes my computer gets extremely slow...video goes out of sync and I can't do anything until I reboot (which takes a full 5 minutes).

Any ideas why this is the case? My rig is listed in my signature. I've had it for less than a year. The sluggishness seems to be triggered by any kind of video being watched, both online (youtube) and offline (mpg, avi, wma.)

Any ideas? This is really annoying.

Rig:
Dell XPS M1330; Intel Core 2 Duo CPU=2.10Ghz; Windows Vista x32; 4.00 Gb of RAM HL-DT -ST DVD +-RW GSA-S10N; 300Gb HD; 13" Plug/Play m1330 Display Nvidia GeForce 8400M GS