I would open Task Manager to see what sort of activity is happening that might explain the problem.
I have been there. It seems that lets say if I open Win Explorer I get nothing for the time (~ 5sec) and then explorer comes up on monitor and at same time I get explorer.exe at about 10% and csrss.exe at around 7%. Nothing stands out to me.
I don't have another XP machine to cross check with. The processor goes to around 75% as there are a few more processes
at 1 or 2 % for a short time ( .5 to 1 sec).