Most of you have probably used this tool before. It's been around for a while and pops up automatically if your C drive starts to get low on free space. If you see this prompt It's usually a good idea to run the tool as it generally means you have less than a few hundred MB's free.
The Disk Cleanup tool helps you find and delete unnecessary or optional files and folder from your hard drive(s) like internet cache (aka Temporary Internet Files), redundant files from software installations and error reports.
I noticed that the free space on my C: drive was getting low this morning so I ran the tool and found a whopping 7.77GB of temp files and error reports clogging up the pipes.
To run the tool in Vista click on the Vista jewel (bottom left) or hit the Windows key on your keyboard, type "Disk Cleanup" and hit Enter.
On XP click the Start button, type cleanmgr in the "Run" box and hit Enter.
Want to read more?
Wiki page on the Disk Cleanup tool: http://en.wikipedia.org/wiki/Disk_Cleanup
Overview of the Disk Cleanup tool: https://support.microsoft.com/kb/310312
How to automate the Disk Cleanup tool: https://support.microsoft.com/kb/315246