could start with ATC Cleaner , thats free .
Then d/l microsoft essentials get the latest update , and run that .
Then uninstall all the the software you dont use .
But to start again and format your drive you really need a windows disc .
I know microsoft had free d/l for windows 7 beta version up somewhere , you could try that , thinks its finished tbh .
If you do try and clean it up yourself , dont pay for any software .
Major geek or source forge are a good place to look , or you can always type ' open source ' in the search box before the program your looking for and I'm sure you'll find a free one
edit
Invalid user's advice is a better way to go about it though