Thanks for any valuable suggestion and help.
IBM T30, 1.8G, 512MB, Win2000 Professional. System updated, Norton Antivirus is up-to-date. No virus found.
Symptom: suddenly the laptop freezes itself. All the running softwares just stand there still. The mouse can be moved freely but no response after clicking on different application softwares. This symptom lasts for about 20~30 seconds. Then everything is back to normal. Seems that nothing happens at all. It can remember the mouse click during this "sleeping moment". By the way, the CPU is not over-hearted at all because only a couple applications are runing, such as Word, Excel, Acrobat, Outlook
Very weird. Can some expert give some suggestion besides reinstall the system. Thank you very much.