hi vikas chowdary
tell about your pc specs,graphics card or integrated chipset.the possible reason is windows 7 support only 945g or higher chipsets
here is a list of supported chipsets
4th Generation Intel® Core™ Processors
4th Generation Intel® Core™ Processors with Intel® Iris™ Pro Graphics 5200
4th Generation Intel® Core™ Processors with Intel® Iris™ Graphics 5100
4th Generation Intel® Core™ Processors with Intel® HD Graphics 5000
4th Generation Intel® Core™ Processors with Intel® HD Graphics 4600
4th Generation Intel® Core™ Processors with Intel® HD Graphics 4400
4th Generation Intel® Core™ Processors with Intel® HD Graphics 4200
3rd Generation Intel® Core™ Processors
3rd Generation Intel® Core™ Processors with Intel® HD Graphics 4000
3rd Generation Intel® Core™ Processors with Intel® HD Graphics 2500
2nd Generation Intel® Core™ Processors
2nd Generation Intel® Core™ Processors with Intel® HD Graphics 3000/2000
Intel® Core™, Celeron®, and Pentium® Processors
Intel® Core™ Processors with Intel® HD Graphics
Intel® Celeron® Processors with Intel® HD Graphics1
Intel® Pentium® Processors with Intel® HD Graphics1
Intel® Mobile Processors
Mobile Intel® 4 Series Express Chipset Family
Mobile Intel® 965 Express Chipset Family
Mobile Intel® 945 Express Chipset Family
Intel® Express Chipsets
Intel® G41/G43/G45/Q43/Q45 Express Chipsets
Intel® G31/G33/G35/Q33/Q35 Express Chipsets
Intel® G965/Q963/Q965 Express Chipset
Intel® 946G/GV/GZ Express Chipsets
Intel® 945G/GC/GT/GV Express Chipsets
if you don't know which one is your chipset then download "Intel® Chipset Identification Utility" from official intel site