Do you notice any slowdown when this occurs?

I have a similar "problem" under W2K. Windows creates a virtual memory swap
file when started, and expands it as needed. Unfortunately, the expanding
swap file causes the machine to slow down, and a restart is eventually
required. (This can take a few days to a week or more, depending on how
"hard" you push the OS. I tend to have a lot of apps and files open at once,
so I have to restart as often as every four or five days.)

It's also possible some freaky malware is running. You need a utility that
shows everything running on the machine, such as Process Explorer.