PP If I remember you are on Vista, so why not try a system restore.?
you would not loose your personal file stuff just restore back to a time when YOU knew the system was working well, any program that you have installed after your restore date may need to be reinstalled but your files and information will be reverted.
If you are on Vista then you should have a "data" partition where the pre installed software was put by the manufacture (unless you deleted it) or had a custom build system in which case you should have been asked to burn an image restore set of disc's.
Do a clean out old cookies and unwanted programs then defragment the drives, then see how the speeds are, I only have one other thing to say and that was told to me by our company IT
"A computer is only as fast as the slowest part in its operation"