A windows update I was doing failed at the "Configuring Updates" stage. After that it said it was reverting changes, but I later found out that the update took over 3 GBs of disk space. Windows Clean Up tool doesn't help.
3Gb is a lot of space by anyone's standards, however, excessive disk space usage by Windows Update is one of my pet moans (I'm seemingly the only one to ever mention it though). What I find is that following a disk clean (make sure you use the clean system files option as well) is that free space returns sometimes a few days, sometimes a week or so later, but it always returns. This months updates used around 1gb of space but nearly a week later and I'm actually 1.1Gb up on the deal.
I'm surprised others don't mention this but there you go... whether yours will return due to a failed update I can't say for sure, but do run the full system files clean and see what happens. A browser clean does no harm either, I usually combine the two following updates and then finish with a defrag of the HDD.
72.9GB start
72.6GB before restart, installed 3 updates, 1.4MB
72.4GB after restart, installed 2 updates, 3.6MB
72.2GB before restart
72.3GB after restart, installed 2 updates, 4.4MB
71.9GB before restart
72.0GB after restart, installed 2 updates, 8.4MB
71.9GB before restart
72.0GB after restart