I read about this thread in this month's Digit issue today (Yeah, I bought it 20 days late). This problem is because Enhancer is a very old plugin and Vista's UAC behavior conflicts with it. It uses a registry entry to keep some of its settings, and that registry entry's permissions don't allow a non-elevated program to access it. If you run Winamp as Administrator, you will see that you don't get that registry error.
I know you already solved it, but IMO, a better solution would be to change the permissions of the following registry entry:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Ioscasoft (if you use Vista 64-bit)
HKEY_LOCAL_MACHINE\SOFTWARE\Ioscasoft (if you use Vista 32-bit)
I am using this on Windows 7 64-bit, and I reckon Vista's location will be the same, but if you cannot find the above entries, try searching for "Enhancer" in the registry editor.
Right click on the entry, select Permissions and give the group named "Users" Full Control.
There is a related thread here:
http://forums.winamp.com/showthread....hreadid=221072