How can I "start fresh" on my iMac?

[iMore Question]

My iMac is over four years old. It's steady, but it has become SLOW. When I have tried twice to wipe it clean and restore from a TimeMachine backup, it doesn't improve the performance. I'm guessing something in the TimeMachine backup is part of the problem. How can I start fresh, get my computer moving quick again like it did years ago, and still get all my files and applications back with minimal work? Thank you!

 

[agarwal.apar]

Just upload all your "important" docs/pics on cloud(Skydrive/iCloud etc). Wipe your iMac and DONT restore from Timemachine. Once your iMac is setup, pull your docs from iCloud and install the apps again from App store etc.