Hi sawani_bade....
First of all, I would consider making a back up of all nessesary files before doing anything else. Make sure to scan everything you put into the back up for virii.
I would then seriously consider doing a complete re-install of windows, and as soon as you have, do a complete 'windows update'
Every windows machine becomes sluggish over time, and when it gets unbearable, you have to manually weed out un-needed files, and run registry shrinking tools to claw back some performance. Sadly doing this often back-fires.
When my PC becomes sluggish to the point that I can no longer bare, I just do a re-install.
Its a shame windows has this issue... linux appears to be free from this to a certain degree.
Good luck anyway.