First, create a System Restore point, then back up your registry using a tool such as ERUNT.
Well you have a few options, firstly I would boot in to safe mode and try from there, if the problem still persists then look at the permission of the key you're trying to delete (right click on the key, choose Permissions). Take ownership and add Full Control for Everyone, then try again. BE CAREFUL to not reset a high level key (such as HKLM), only target the key you are looking to delete.
Next I would try using subinacl.exe, which traditionally comes with the Windows Resource Kit Tools however Microsoft has made the XP version available as a stand alone download here.
Syntax from the command prompt would be as such: