1) Turn system restore down to the bare minimum; by default it uses 2gb of space to store it and its a breeding ground for viruses; the minimum is 250mb and thats more than enough for most people; I have it turned off.

2) Turn the Windows update to notify but not download

3) Turn off "Allow Remote Assistance"

4) Grab Spybot S&D and Spyware Terminator for Spyware and AVG/Avast for a free anti-virus; I prefer avast.

5) Grab CCLeaner to clear out the registry of any unwanted crap.

6) Delete everything from the C:\Windows\prefetch directory

7) Make sure all programs are upto date and clean that sucker out.

This is what I do to make $75-90 at work :P And depending on how long it takes, sometimes it can take an hour sometimes it can take 2-3 hours pending on situation.