The best answer i can give you is... NB : ALL SOFTWARE INSTALLED WILL NEED TO BE RE-INSTALLED
1) clear up to at least 12Gig of space on drive C:
2) Get an installation disk and re-install your OS
3) When prompted to format click " leave partition intact"
4) After all that is done you will find all your Documents in C:\windows.old
Windows accumulates old files and unnecessary files with time and by doing so it does not recognize if that file is useful or absolute, hence forth it will try and read the file so as accomplish a task, wich decreases your processing speed and your anti virus might clean a file and corrupt it so it is always best to start over cause its just 40 minutes out of your time and your machine is back to peak perfomance...
God Bless and Good Luck