sound to me like you need to do a good clean up to start with, a hard drive needs plenty of free space to operate at a decent speed, get yourself a clean up program or two, i use Clean up! and CCleaner, they will get rid of all the temp files all microsoft software just loves to leave behind.
Make sure your hard drive is less than half full, over 50% full and they really slow down but if you really need everything i would look at an extra drive