Cleaning up a failed SQL installtion can be a real bear, but sometime you can't reinstall until all traces of a failed installations are removed.

Here are some hints:

1) First go to add/remove programs and remove any SQL components that are listed.

2) Open regedit and delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server

3) Delete the Program Files and Program Files(x86) Microsoft SQL Server folders

4) Go into local users & groups. There there may be custom users installed by SQL. Remove them.

Reboot and look again for the above items to make sure they weren't added back.

Good luck!