Came across a nice tip when I was researching an unrelated problem today. I discovered that I could use dism to cleanup Vista/7/2008 Service Pack install backup files to reclaim space!
Warning!
The service pack can’t be uninstalled after this operation is completed. This operation will remove backup files created during service pack installation. Use /hidesp
switch to hide the service pack from Installed Updates.
dism /online /cleanup-image /spsuperseded
It took about 45 minutes to run on a Server 2008 R2 SP1 system I have in a virtual lab with 2vCPU and 2GB RAM. YMMV.