There have been recorded cases when even the most sophisticated uninstall applications leave traces of the software they were supposed to remove on your computer. It happens, for example, when a virus, or spyware or, even, malware has infected your software. In these situations, manual removal is your best (and sometimes only) bet.
It is somewhat involved, granted, because you have to remove the offending pieces of code from several locations, starting with the registry, going into the start-up area, continuing with the executable files inside the C:\Program Files folder, and then checking all other potential folders in use.
The place to be the most careful is the registry: you may â€“ by mistake â€“ remove something that your computer needs unconditionally if it is to boot. Make sure you know what you’re doing and, better still, make a copy of the registry as it exists BEFORE you make any changes. Your computer will thank you for it, and so will your nervous system and general sanity.