My computer is running sluggish lately?

my laptop is about 4 or 5 years old now and just recently it's began to run a little slow. When I type it will take a few moments for the letters typed to catch up (it can't keep up with me). Also in firefox, sometimes the pages freeze up for a few seconds before loading . I tried running a defrag but it says it can't because it has to 'check disk' or something. I'm running norton and I've had no problems out of it at all. My harddrive still has 3GB of free space. Is there anything I can clean out that might speed it up a little?