Actually I've never really seen that much performance difference running a defrag. There a so many other things that will increase performance a lot more.
1. If you are getting low on virtual memory, you should probably up the setting. This is something you can do right now.
Right click on My Computer
Select Properties / Performance tab
In the Virtual Memory section, click on the Change button
I don't know what it is set at or how much memory you have, but typical settings are 1.5 to 2 times the amount of memory. Then click on set. Reboot.
If this isn't enough then make it larger.
2. Add more memory. It's pretty inexpensive these days and can only make the system perform better.
3. Clean out the TEMP directory. This is the user's temp directory, not the IE cache. I've seen many computers where the owner was ready to get a new one. Just cleaning this out and rebooting made a huge difference in performance.