I'm getting a weird error:

There is not enough disk space on C:\WINDOWS\$NtServicePackUninstall$ to install Service Pack 3. Setup requires a minimum of 4 megabytes of free space or if you also want to archive the files for uninstallation, Setup requires 4 additional megabytes of free space. Free additional space on your hard disk and then try again.
That's all fine and dandy, but my drive's got 54.5GB free.