If you have removed the Backup Files and Directories user right from the Administrators group, attempting to install a Service Pack will generate:
Service Pack Setup could not backup registry key HKLM\System\CurrentControlSet\Windows\ to file d:\$NtServicePackUninstall\reg00001.
1314: A required privilege is not held by the client.
To fix the problem, you must edit the policy that is being enforced on this computer (Local, site, domain, or organizational unit):
1. Start / Run / secpol.msc / OK.
2. Local Policies / User Rights Assignment / Backup files and directories / Add.
3. Double-click the local Administrators group.
4. Press OK.
5. Do an immediate group policy refresh.