It turns out that after installing SP1, the process leaves behind ~800MB of original Windows files to allow uninstallation.
After filling up my old laptop last night a quick search found this article pointing to a cleanup utility that sits in the Windows\System32 folder.
To run the utility, start a command prompt as an admin (right click | Run as Administrator), change directory to the system32 folder, and run ‘vsp1cln.exe’.