Go to this key in the Registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Run
In the right-pane, delete the entry for the offending program.
You might also check this key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\Curre ntVersion\Run
And delete the program if it's there.
After that, just go throught the Registry and delete any keys that make reference to the program you removed. It's safe if you backup the Registry, I never do.
There's also some Registry cleaners, but I've never used them either.
You might also check and make sure there's not a shortcut to it in the Startup folder either.