Help! I cann't get my computer to run like it did when it was new.?

I have Spybot Search & Destroy and Ad Aware SE Personal, which I use to scan for adware and spyware, I have CCleaner and Eusing Free Registry Cleaner, which I use to scan and clean my registry, I have the latest version of AVG Free, which I use for viruse detection, and lastly I have Kerio Personal Firewall running 24/7. I run these programs daily and remove any issues they find. When I first boot up my computer it works fine but, after about 30 min. it starts to slow down and the longer I stay on the slower it gets untill it freezes up. I added all of the above programs to try and fix the problem, and they do help. But, the only way I have found to avoid this is to reboot every 30 to 40 mins. There must be a better way. Please Help! Thank you.