It is almost never really necessary to "start over" as the machine can be "cleaned" without a fresh install.

Have you simply removed all the programs that you no longer need via "add remove programs"?

Have you removed all those old internet files and programs in the temp folders?

Have you removed all internet parasites using programs such as NoAdware and Spybot?

While not necessary, there are programs that can find registry entries for which no program exists and even clean those for you.

What is the problem you are trying to address?