CCleaner, The Tool which all Windows users need!
CCleaner is a tool I always install Windows, and use regularly to make sure my machine is running quick and is clean.
It basically removes unused temporary files, logs, application data cache data and junk files. It has support for many applications, such as; All major web browsers, Office, Windows files, Flash, VLC, MSN, AntiVir, and the list goes on.
Apart from cleaning old files, it has an excellent tool for finding unused registry entries from old application left there after they were removed. Word of warning, it could always been dangerous changing things in the Windows registry, but I have never had an issue using this tool.
CCleaner is also an excellent tool for removing software and managing which software starts on boot up, a far better solution than what Windows is offering.
There are many solutions like CCleaner, but most of them cause damage to your system and slow it down, basically Junk-Ware! CCleaner gets the job done, always being updated to support more applications, and will improve your system's performance. On your first clean up, it will likely find over 2GBs of junk data!
CCleaner can be downloaded for free here!

CCleaner is a freeware system optimization, privacy and cleaning tool