Left Shift does not work for multiple selection, works for capitalization

I have a Dell Latitude D830. The OS is Windows XP SP3. The left shift used to work fine. But recently it does not function when I try to select multiple files in Windows Explorer, Outlook, and some other application software for more than 90% of the time. But it always functions well for Capitalization. The Right Shift key functions for multiple files selection for most of the time.

It’s annoying. Can anybody help me?

Thanks a lot!

Paul