My notebook was down to the last 800MB of disk space. Amazingly I just claimed back 17GB through Windows Vista Disk Cleanup!

Wow!

The big wins were:

• It turned off Hibernate functionality for me - which I never used (3.4GB)
• It got rid of a bunch of dmp files from when I had some driver issues causing stability problems a few months back (1GB ish)
• Temporary files (2GB ish)
• Shadow files and restore points - a huge 8GB!!!!!!!!!

Very sweet (although I wish Vista had advised me to do this a while back).