cCleaner is an excellent utility which gives you back space on your harddrive. On one machine I almost got back 500mg when I ran this utility the first time.
Filehippo is an excellent freeware site for getting free Antivirus such as AVG and other goodies.
If your machine has been hijacked by some nefarious spyware you should first get Hijackthis. this utility gives you a listing of your registry and then you can get help with some sites which help yo udetermine who the bad boys are .
This utility will give back to you disk space taken up by cookies, temporary files, and other junk that accumulates on your machine over time.