Explorer is actually a user process in XP. It has to compete with all the rest of the processes for system resources. Somtimes there just isn't enough for explorer to do all that is being asked of it. I would investigate how many background processes are auto-starting and running full-time.
Sysinternals free Process Explorer: http://www.microsoft...ssexplorer.mspx
And free WinPatrol: http://www.winpatrol.com/
are very helpful in this regard.
There are several malware exploits that can cause this behaviour as well. If you suspect malware, please go to the malware section of the forum.
It would be interesting to see if the behaviour presents in a newly created user account. If all is well there it is not difficult to switch: http://support.micro...om/?kbid=811151