Don't worry, help is at hand in the form of some clever free bits of software that can help you manage your drives:
CCleaner (or crap cleaner) is an excellent product, which when run on your machine will get rid of all those temporary files that applications are so keen on leaving all over your hard drive. It is worth checking the settings before as you may not want it to remove things like your internet browsing history (or maybe you do). CCleaner also includes a registry repair process which removes junk from your registry.
WinDirStat will helpfully scan your disk drives and map the files on it and their usage. The graphical display makes it really easy to see what is hogging your disk space.
|WinDirStat maps your disk drives|
The PC Decrapifier is for people who have just got a new PC. running this application will help you get rid of all the junk that PC manufacturers are so keen on hoisting on their customers. Highly recommended.
If you have any recommendations for useful disk software please add a comment, I would love to see what people are using.