rundll32.exe Application not Found error

I have a Dimension 510 running Windows XP Home Edition.  I am unable to run any program that has a .exe extension on it, and cannot access my Control Panel commands, and get the above error message.  When I attempt to run a program it states "Choose Program you want to use to open this file".  I had a trial version of Mcafee virus software running, and when it expired, I renewed for one year; however, the the new subscription is not being verified by the existing sofware, and I cannot uninstall it, due to the rundll32.exe error, nor run any new downloads.  I tried a free trial scanning program from the web, which removed some spyware programs, but this did not help.  I don't want to have to wipe out my entire hard drive with Restore function, as I have a ton of personal data on the hard drive, and no easy way to back it up.  I searched for the rundll.exe file in the system 32 folder, and it was there; however missing the .exe extension.  Can anyone help?