one of my laptop just had this problem

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.