back up registry failure

i am using windows xp. i get automatic updates alot and the last one that i received was to install Service pack 2. when i click on my automatic updates i click the recommended install, then the install wizard appears and i click install to install service pack 2. then a window pops up saying that it is backing up my files, somewhere in the middle i get an error message that says: "could not back up registry key HK CR:\.DVR-MS to file C:\WINDOWS\$NtServicepackuninstall$\reg00013. 5:access denied". i do not understand this and i dont know what to do. can someone please help me. i am having trouble sometimes with my computer freezing up and stuff and i go to sites to ask about this and it says that i need to install the latest version of service pack, which i cannot do. if someone could help i would greatly appreciate it. Thanks so much, Desiree Cave