I am troubleshooting one of my lab computers (a netvista workstation). The windows xp install has gone very very slow and is making the machine crawl after about 40 mins or so after startup. Sometimes this happens after an hour, sometimes after 2 hours. Doing a reinstall is out of the question, too many important programs running on it for which we currently don't have installation disks. I tried to use Task manager and later Process Explorer to see what is causing the sudden memory hogging, but have not had any luck yet. System is behind a firewall and there seem to be no viruses/spyware (I did an extensive check).
I was wondering if anyone knows a good (free) memory usage monitoring program. Of course it should not be heavy itself

I am specfically looking for a "widget" kind of program that can run on top of all other programs, and that has a graphical output of memory usage (pie chart, regular graph like the tast manager etc) - i.e something that I don't have to look at all the time, but if there is a spike or anything, I won't miss it if I am working on something else.
Is any such program available? Any info is appreciated.