Ccleaner

Ccleaner is one of the must-have tools for your system, especially if you go online frequently. Crap cleaner or Ccleaner is a software from Piriform Ltd. There'll be a lot of temporary internet files stored in your PC whenever you browse online. Sometimes it occupies a lot of memory. Once when I cleaned my friend's laptop using this, a whopping 3 GB of temporary files were deleted. There are other features like Registry Optimization,editing your start-up programs etc. Here's what they (Piriform Ltd)  say about Ccleaner :

CCleaner is a freeware system optimization, privacy and cleaning tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware! :)