|Run disk clean up first. Delete caches if using other than IE.|
Use disk compression to make space. It may speed up the system. Compressed data can be transferred faster than uncompressed data in most cases.
Consider using the add and remove programs feature to remove un-needed applications.
Using a usb external may allow you to free up space.
Adding ram may help speed it up.
I tend to suggest to people that if they felt the computer ran well when new then return it to OEM state with the recovery disks. I have a 10 year old computer that I do that to every few years and it works great still.
"The era of big government is over," said Clinton 1996