Have you ever cleaned the inside of the case and the fans?
If not, then do it but with care. I used a dyson to do mine and cleaned all around the vents, CPU cooling vanes and graphics card, made a remarkable difference it did.
Still not going well, You may have loads of stuff starting up when you boot it. when started clcik on start, select run and type in - msconfig
This will bring up a window, select the start up tab and look to see whats loading when you boot up, de-select anything you know you do not need. Not to worry here as if you need the program just open it as usual.
next thing to do is to run the disk clean up and defrag the hard drive.
To clear out any junk files do the following.
Click start, then the search option.
1. Search for *.tmp when finnished select them all and delete them
2. Next search for *.log and do the same
3. Next search for *.log and delete them as well.
You must type in as shown with the *.
Post back your result as you may need to run some anti spyware just in case...