How can I completely uninstall SQL Server?

Learn more about how to un-install a SQL Server servicepack and go back to the original version. 

A. If the standard un-install utility doesn't do the trick for some reason:

Remove the <sql> directory and everything under it
Remove the <sql> dir from the path (use control panel/system for this)
Remove the SQL registry entries using regedt32/regedit. These are :-

(All versions)

• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSSQLServer
• HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer

(6.0 and above.)

• HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSDTC
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SQLExecutive

(7.0 and above)

• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SQLServerAgent
• HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server 7
• HKEY_LOCAL_MACHINE\Software\Microsoft\MSSQLServ65

If you have lost permission to any of the registry keys, then log on as administrator and take ownership of them starting at the highest relevant level. Use regedt32 and check the box to take ownership of all subkeys as well.