I'm at a loss

I'm running windows XP professional, 64. Yesterday I came down with the "Security Protection" virus which I assumingly removed by following the instructions on a youtube video.. However, once I deleted the defender.exe file (which I renamed) I tried to run malwarebytes but got the "windows cannot access the specified..." error. So I came here.

I've uninstalled and reinstalled malwarebytes several times from several different download links. None work. The first time after an install, the scan runs for about 5 seconds, and then the program closes without any error messages or anything. Attempting to run it again results in a "windows cannot access..." error. I downloaded superspybotremover, same thing.. first time runs about 5 seconds, then closes with the error. Since I run 64 bit windows I read that I shouldn't run combofix or the go-to log file..

Any help would be very appreciated. I've got the computer running enough to get on the internet but its very very very very very (like 1997 dial-up) slow and none of my games run.