You have to do some detective work.
A well written program (or at least uninstaller) will completely remove references to itself in the registry, delete its unique files and folders, and remove any associations.
This means that you might find some remenants of programs in the registry (or in a directory on the hard drive, etc), but don't count on finding all of them!
Good luck! Mudskipper
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Groucho said it best- "A four year-old child could understand this! Quick! Run out and find me a four year-old child: I can't make heads nor tails out of this!"