You need to record what went wrong - error messages, etc. For example, here is a quote from just one of the several links off the MS link I gave above. Until you give more precise idea of what happened, it is hard to find the right solution.
"How to determine whether the registry update has failed
Click Start, click Run, type %windir%\svcpack.log, and then click OK. The Svcpack.log file opens.
On the Edit menu, click Find.
In the Find what box, type DoRegistryUpdates failed, and then click Find Next.
If there was a registry update failure, a "DoRegistryUpdates failed" error message is displayed in the file. This indicates that an issue is preventing the registry update. This error message resembles the following:
xxxx.xxx: DoInstallation

oRegistryUpdates failed.
xxxx.xxx: Access is denied.
xxxx.xxx: Message displayed to the user: Access is denied."
Here is a link to what to do about that and download the SP3 again - note this is the FULL service pack.
http://support.microsoft.com/kb/949377/