Why Is My Computer Lagging (continued?

OKay
MY computer:
Intel(R) Pentium(R) D CPU 3.00GHz:CPU
3.0 GHz Performance Rated at: 3.60 GHz:CPU speed
Microsoft Windows XP Professional Service Pack 3 (build 2600):operating systim
GeForce 6800 GS:video card
Video RAM 256 MB 743.0 MB
Hardware T&L Yes Yes
Pixel Shader version 3.0 3.0
Vertex Shader version 3.0 3.0
IDT High Definition Audio CODEC;sound card
86.6 GB on current Hard Drive
I’ve ben playing my video games for a long time no lag just all awsomeness but one day i went to bed and turned off my comp i woke up in the morning and all my games were laggymy internet speed is high and ive updated everything,Also i dide registry checks,malware checks,virus scanning and Nothing worked[][] direct X,video card,windows updates, graphic card updates[][],i even cleared a bunch of games on my profile but still laggy?? i need help