Imac - How Do I Start Again From Scratch

[siam]

My iMac has been running for 5 years non stop and now its starting to show the spinning beach ball on a regular basis , ive done the maintenance things ( inc Drive Genius / Disk Warrior Etc / repair / Verify etc ) but the problem still exists , I do regular Time Machine backups , and currently run 10.6.8 but at the moment I do not want to upgrade to Lion. So I thought it was now time to start again from scratch . I am wondering what's the best way to go about this , and what happens to all the paid applications Ive currently got . If I just re install from Time Machine then may be I will just return to the same problems I'm now having , is there any simple way I can start from scratch using the original CD's that came with the iMac and some how reinstall the paid applications ive collected over the years . As ive never done this before Any advice would be most welcome