[dba-SQLServer] Nuke All Traces of SQL Server

A friend has a problem that I seem unable to solve. Using the install
wizards, he has removed SQL 2000, MSDE, Express and SQL 2005, and then
attempted to reinstall 2005. It doesn't work correctly. Something, perhaps
in the registry, is going wrong.

I want to tell him how to remove all evidence that any SQL-related product
was ever installed on this box, so he can start afresh. It's simple enough
to run the un-install and then nuke the directory, but there may be traces
in the registry. Anyone know what they are and where to find them?

A.