It seems a number of people are having problems installing this service pack. There are a couple of solutions (here, here and here) that may work. There is also anecdotal evidence that exctacting the files from the SP executable using WinRAR andf running HotFix.exe manually may work.

Alternatively, I have found that the following works for me:

1. Download the service pack and start the installation by double-clicking the downloaded executable.
2. Once product enumeration is complete click Next at the Welcome page.
3. At the End User License Agreement page, check the box to accept the terms and conditions before clicking Next.
4. Select All Instances, Windows Authentication at the Authentication Mode page and click Next.
5. Click Install to begin the installation.
6. During the install you will be told about pending file operations. Click Yes.
   
7. A further message appears complaining about processes locking files. At this point, stop the SQL Server Full Text Search service and click Try Again. You may also need to click Try Again additional times depending on what SQL Server processes happen to be running.