Try plugging in a different USB keyboard and see if it still does the same thing. If so try running diagnostics on yourcomputer to see if you have a hardware problem. You should have a CD that camewith the computer that has "Diagnostics and Utilities" listed undercontents. Boot form this CD and run the extended diagnostics.
If you don't have the CD you can go to www.support.dell.com
and select "Drivers and Downloads". Enterthe "service tag" (serial number) of the computer. You can thendownload "Diagnostics" .
To boot from a CD disk, power on your computer and tap the key tobring you to the "One time boot menu". Insert the CD into the top CD/DVD driveand then select "Boot from CD/DVD" and hit key. Select the"Run the 32 Bit Dell Diagnostics.
If all tests pass then reload OS ( Operating Software ).