Loads you can do firstly would clean up c drive using the windows tool also remove anything you no longer use. (when installing games you should defrag before and after installation because games fire the files all over the hard drive) then defrag to see if that makes a difference.
Your media files would be better on d drive. Next you can check the "commit charge" on the task manager (lower right hand corner) guidelines state that if amount in use (left figure) is more than 50% of total figure (right hand) you need more RAM.
You can also check which progs run on startup as normally are plenty you don't need access too on regular basis. There is an excellent free prog called startup.exe (use google to find) which lets you control which progs load at startup.
Of course spyware is another problem which can slow pc if you don't have then go onto www.majorgeeks.com and you'll find good freeware there.
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