Hi throbinson and welcome to Vista Forums
Are you trying to install SP1 via Windows Update? If you are, try installing it from the downloaded standalone version (link given below). You will need to save this to a DVD. Once you have done that, you should be able to install SP1 from this disc. Note that this may take a while to complete and you should allow it to do so at its own rate.
Download details: Windows Vista Service Pack 1 Five Language Standalone for x64-based Systems (KB936330)