How to decrease the lag when switching between programs?

I get an extreme lag whenever i switch from program to program. Please tell me how to solve this? I have also checked that am free from anti viruses. All this started after recovering like from back to factory condition from my pc.

Memory is used a lot in games. You might have noticed that when you switch out of heavy graphics game, the windows take much time to load. And the Windows program dosent seems to have this problem cause windows elements are shared between them.

The minimum requirement to run vista is 800 MHz & the recommended is over 1 GHz. So this will cause lag an time. Also videos take too much resources.

Thanks for all the comments & now that i know that this is not a gaming machine that i thought it would be.