It isnt a standalone program- it is part Windows- The only why it could have got "broken" is through Windows Corruption such as caused by Malware or programs like Registry cleaners.
The only way you can fix this:
Back up User data before trying a reinstall operation
1. try a system restore to a point before the corruption occured.
2. Try a "upgrade" install of Vista
3. Do a repair/full re-install of Vista