Reformat Windows. Those cleaners and anti-virus sellers are just ripping people off. If you still want to buy them, I have some WunderHair Toniq that I'll sell ya. Will grow back balding hair in under a week! Only $100 a bottle!
CCleaner(free), AVG(free), and the windows built in defrag whose engine makes up the best aftermarket defrag for windows. Although it was written by scientologists.
The CCleaner guys have a new product DeFraggler. It is free too, but I have not used it, so I cannot recommend it.
No such thing. Total snake oil. CCleaner is a good maintenance problem that deals with a few specific problems in the registry and is worth having, but those programs that claim to "clean your registry" and speed up your system several times over are total snake oil.