You do not have to re-install your O.S. I am assuming that you are using Windows. Here is how you can fix the problem:
What you have to do is use regedit to delete all traces of any reference to that program.
To do this go to your start menu and type in regedit at the bottom in the box and press enter in Windows Vista/7 or if using a older version of Windows, go to the start menu, run, and then type in regedit there.
Next you have to go to each HKEY folder in the left panel.
Yours might be a little different then mine but it will look something like this:
HKEY_CLASSES_ROOT
HKEY_CURRENT_USER
HKEY_LOCAL_MACHINE
HKEY_USERS
HKEY_CURRENT_CONFIG
Start with HKEY_CLASSES_ROOT and find the folder of the company that makes your VST. Delete the whole folder AS LONG AS YOU DON'T HAVE ANY OTHER VST BY THE SAME COMPANY!. If you have another VST by the same company, go into the folder instead and delete the VST folder that don't work.
Next go to HKEY_CURRENT_USER and then the Software folder. Check for the VST in there and delete the folder if you find it.
Next go to HKEY_LOCAL_MACHINE and then the Software folder. Check for the VST in there and delete the folder if you find it.
You do not have to go to the last 2, HKEY_USERS or HKEY_CURRENT_CONFIG. That is it. Restart your computer and then try to re-install your VST
With some VSTs/programs you might also have to delete another folder. If what I just told you to do above does not work, I will tell you what other folder to delete.