Sometimes a software uninstall might not complete fully on a Windows system, and you'll be left with an entry in Add/Remove Programs, even though the program files are no longer present. Attempting to remove the program from that list again sometimes will generate an error, and the entry will not be removed, leaving you with an annoying orphan. IntelliAdmin has a freeware program called Add/Remove Programs Cleaner (link) which removes entries from that list.

Important Note: This tool does not do anything toward actually uninstalling a program's files, shortcuts, or registry or profile data. It only removes the item in the Add/Remove Programs list, and it should only be used on a program which you know has been uninstalled, but which Windows won't remove from the list when you try via the normal means.

The Cleaner works on Windows NT, 2000, XP, 2003, 2008 and Vista, and may work on Windows 7; I believe it does not work on Windows 98 (seriously, you aren't still using that, are you?) It does not require an installation; it is simply a single executable file that you run.