Computer performance continues to fall

What Happens with my computer. Every once a week my computer performance is decreased.
Specification:
DualCoreE5300
3 GB RAM
200GB hard drive – +
VGA ATI HD5450
PSU500Watt (still good)
Though already done a lot of Tweak. Including Tweak to boot who wear Ntosboot. Its performance is getting slower by the day.
I'm sure not a virus, I just use Kaspersky as antivirus.
Already use GAMEBOOSTER (no effect)
Additional LOADING very long game, but when it is reinstalled quickly.
His game is loading VERY OLD even up to 10 minutes, namely:
Left4Dead1 (7Menit-+)
LEFT4DEAD2 (10 minutes – +)
FAR CRY 2 (Sometimes Freeze)
Though in my friend that his lower processor is fast loading only, Even months of not re-install.
