I just did this with mine. First off, do you have anything like the BBC iPlayer/ITV player/Sky player ? If so, delete them immediately, and then check in your program files for something called "Kontiki" or "Kservices". If you have them, delete them. When you sign up for the BBC player/ITV/SKY etc you sign up to them being able to use your computer as a host for their content. So, when you download last nights CSI or whatever, the next person that wants it comes to your computer, rather than their servers. It's a real liberty, and keeps your internet connection going morning noon and night. You can search for an application on the internet called "killkontiki". It's a VERY popular subject on the internet right now.
Norton antivirus is good, but constantly uses up space.
I use a program called "Advanced Window Care V2" which is free from the developer, and it cleans up all the unnecessary crap from your computer, and also shows you where stuff lurks that you probably don't know is there.
You can get it here
Since getting rid of the iPlayer, and stopping some of the crap from yahoo, apple, adobe, etc my computer runs about twice as fast