I use 2 registry cleaners on a regular basis and it keeps my windows system running as smoothly as possible. Let's face it the windows platform is just the worst piece of crap but we can't all afford a mac. the registry cleaners cleans out all the unused left over data from removed software and also defrags the registry, deletes obsolute files, temporary files and broken shortcuts. In a nut shell, it will clean out the crap out of windows lol. I use:
Registry Booster
System Cleaner 5