Note : I'm a self-admitted noob on computers and hardware, please keep it simple
I recently bought a new computer from NCIX and 2 months later my FPS in all my Source games (the only modern games I have on my PC, CS 1.6 still works fine

) just took a nose dive in preformance. FPS was more than halved in most cases. Up until this point I had easily over 100 FPS on maxed out settings and never dipped below 70 or 65. Now its as low as 30-40 in most cases with graphics severly toned down. I had not changed any hardward or messed with any drivers when my FPS decreased, just one day it decided to die on me.
--System Specs--
These are taken from the receipt from when the PC was ordered :
-Intel Core2Duo E8500 LGA775 3.16GHz Wolfdale 1333FSB 6MB
-Gigabyte GA-EP35-DS3L ATX Motherboard
-2x Corsair XMS2 2x1GB DDR2-800 Dual Channel Memory
-GeForce 9800GTX+ OEM 738MHz 512MB GDDR3 2.2GHz PVI-E
-Corsair TX750W ATX 12v 60A 24PIN Power Supply, 120mm Fan
-Arctic Cooling Freezer 7 Pro LGA775 2500RPM 45CFM
-Seagate Barracuda 7200.11 3.5IN 500GB SATA2 8.5MS 7200RPM 32MB Cache Hard Drive
-HT Omega Striker 7.1 7.1 Channel PCI Sound Card
-Some random case, 120mm Fan
-Windows Vista Home Premium Edition 32Bit OEM
Again, nothing that I listed up there had been even touched before I got this problem. The first time I even opened up my PC was when this happened.
Any help is welcome, thanks in advance