3-year old lappy, core2duo, 2gb.
Downloaded Microsoft Process Explorer. It showed that one of the svchost was using a lot of memory, and the memory usage kept increasing until system ran out of physical memory. Eventually the laptop slowed down to a crawl.
There were many services running under this particular svchost, such as automatic update. Disabling most of the services didn't help.
Scanned the lappy using Norton, Malwarebytes, didn't find anything.
Eventually I ended up re-installing XP.
Now it's running pretty fast.
