Problem with right-click in W. Explorer

I have had this problem in both in Windows 2000 and WinXP. After a while (month or two) after running my computer normally without a problem, suddenly my computer takes a long time to react. When you right click an item in Windows Explorer it takes about 10-15 secs to pop up the menu. The same thing happens when you press the delete key for deleting files – 10-15 secs.
I am currently using WINXP Pro. on a Compaq P800 with 256MB Ram.
I have not added any programs or hardware lately. I am connected to a Win2000 AD network.

Does anyone have any ideas??