Problem running games on low settings even....

hii there.... I am facing a svere problem.... I am having serious stuttering and lagging issues with games...
yesterday I installed Assasians Creed 1.. It worked fine and I played for about 1 hour on ultra settings.. After some time I again started the game but then It starts lagging..... I changed the settings to low but have no effect... I then installed Street Fighter X Tekken but again it also doesn't worked and stutters a lot....
Also I Cant even play MW3 on even low settings without any special effect.... Please help... I don't what the hell is causing all this problem... Here are my system specs..

Operating System: Windows 8.1 Pro with Media Center 32-bit (6.3, Build 9600) (9600.winblue_gdr.131006-1505)
Language: English (Regional Setting: English)
System Manufacturer: Dell Inc.
System Model: OptiPlex 755
BIOS: Phoenix ROM BIOS PLUS Version 1.10 A10
Processor: Intel(R) Core(TM)2 CPU 6300 @ 1.86GHz (2 CPUs), ~1.9GHz
Memory: 1536MB RAM
Available OS Memory: 1518MB RAM
Page File: 1306MB used, 1619MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
DxDiag Version: 6.03.9600.16384 32bit Unicode

Display Devices
---------------
Card name: NVIDIA GeForce GT 220
Manufacturer: NVIDIA
Chip type: GeForce GT 220
DAC type: Integrated RAMDAC
Device Type: Full Device
Device Key: Enum\PCI\VEN_10DE&DEV_0A20&SUBSYS_00000000&REV_A2
Display Memory: 1735 MB
Dedicated Memory: 976 MB
Shared Memory: 758 MB
Current Mode: 1024 x 768 (32 bit) (85Hz)
Monitor Name: Generic PnP Monitor
Monitor Model: Acer AC713
Monitor Id: ACRAC04
Native Mode: 1024 x 768(p) (84.997Hz)
Output Type: HD15
Driver Name: nvd3dum.dll,nvwgf2um.dll,nvwgf2um.dll
Driver File Version: 9.18.0013.3221 (English)
Driver Version: 9.18.13.3221
DDI Version: 10.1
Feature Levels: 10.1,10.0,9.3,9.2,9.1
Driver Model: WDDM 1.2
Graphics Preemption: DMA
Compute Preemption: DMA
Miracast: Not Supported by Graphics driver
Hybrid Graphics GPU: Not Applicable
Power P-states: Not Applicable
Driver Attributes: Final Retail
Driver Date/Size: 12/20/2013 01:26:04, 15230352 bytes
WHQL Logo'd: Yes
WHQL Date Stamp:
Device Identifier: {D7B71E3E-4960-11CF-256F-0D201FC2C435}
Vendor ID: 0x10DE
Device ID: 0x0A20
SubSys ID: 0x00000000
Revision ID: 0x00A2
Driver Strong Name: oem5.inf:0f066de3ec1dddbe:Section004:9.18.13.3221ci\ven_10de&dev_0a20
Rank Of Driver: 00DA0003
Video Accel: ModeMPEG2_A ModeMPEG2_C ModeVC1_C ModeWMV9_C
DXVA2 Modes: DXVA2_ModeMPEG2_IDCT DXVA2_ModeMPEG2_VLD DXVA2_ModeVC1_VLD DXVA2_ModeVC1_IDCT DXVA2_ModeWMV9_IDCT DXVA2_ModeH264_VLD_NoFGT



P.S. I also have latest audio and video drivers