All games lagging in ati hd 4650

Hello,
Im getting a wierd problem. my PC Configuration is - 3GHZ Core2Duo, 4 GB DDR2 Ram, 320 GB HDD, and XFX ATI RADEON HD 4650 1 GB DDR2 Graphics. I was able to play all recent games with almost nt being lowering settings . The problem starts from 4 days earlier when im going to play assassins creed revelations . its crawling so badly in the default settings I lowered the setting but still same problem . I dowloaded newest driver 11.11 but still same problem arises . i thaught this game will not support in my system so i tried some old games which i used to play in full configuration without any lagging like NFS Hot Pursuit. I got shocked this game also crawlling so badly. Sir I formatte my pc for 5 times in last 4 days. Can I myself solve this problem ? Plz help me out what to do . Im so confused. Plz reply fast.
oh I forget to tell after lagging for 2 or 3 mnts my pc got automatically shutted down . somebody plz help .