It's not spyware/adware/viruses. The HDD is not fragmented, nor is it dieing.
I clean the registry, and defrag, and run spyware/virus checks once/wk. I also check the processes that are running periodically, and clear my startup in msconfig everytime I install or change a program.
The problem is easiest to find if I am playing a music or video file, and downloading or unzipping.
If im downloading a file, or unzipping a rar, and open an mp3 in winamp the sound will slow down noticeably, and the sound will become garbled because of that.
Also, the system likes to lockup for short periods of time if I open IE or MIRC or something.
Another feature of this problem is that if I play a WMV file, and set it to repeat indefinately in MPC, then MPC will lock up at the end of the file and it has to be closed from the task manager. But mpg's, avi's, mkv's all play fine. Although mkv's like to run slow sometimes(sound/video go out of sync).
When im playing an H.264 I can sometimes see a noticeably FPS drop, and hear the slowdown through garbled sound.