Looks like the SP3 update wasn’t tested on MacBook’s. I had a few excrutiating attempts to install and always being told I was 4MB short despite having 20GB free.

There is not enough disk space on C:\WINDOWS\$NtServicePackUninstall$ to install Service Pack 3 Setup requires an additional 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.

Thankfully I found a helpful guide at windowsreference.com which is an easy registry hack and seemed to do the business with no problems.