Your computer may be infected by the Pretty Park virus. You may restart to safe mode and searc for the file Files32.vxd. It is an indication of the virus. For more information, click <a href="http://www.symantec.com/avcenter/venc/data/prettypark.worm.html" target="_blank">here</a>

If your computer is not infected by the virus, you can try to restore an earlier registry file. This may help: From Safe mode, restart to MS-DOS mode, then type "scanreg /restore". Select an earlier reigstry file when the problem didn't exist and restore it.

Furthermore, you can run "msconfig" from safemode and uncheck "Load startup group items" and try to start your computer in normal mode. If it success, there is a program in the startup group causing the problem.
(If the problem persists in safe mode, the above steps won't help.)