While attempting to install Windows XP Service Pack 2, an error pops up complaining that access to a registry entry (most likely in HKCR) is denied. The installation fails, and the update backs itself out.
First, check out the following URL:
http://support.microsoft.com/kb/873148
Second, this one talks about the same stuff, but goes into more detail.
http://winhlp.com/WxSP2.htm
If you cannot edit permissions on the registry keys and you are an administrator, try this:
http://forum.aumha.org/viewtopic.php?t=77968&view=next