I can't speak for Defraggler, but Smart Defrag 4 from IOBit will defrag your page file and other "System" files during a "boot-time defrag". Some defrag program wont do that. Do this after you've set the pagefile to a fixed size. You should use CCleaner too and follow RM's advice about running a thorough disk cleanup.
However, before you do anything to your hard disk, run an error check to detect and correct any disk errors.
1) Error Check.
2) Run a windows Disk Cleanup, including old system files.
3) Run CCleaner or something similar. CCleaner deletes unnecessary files and can also scan for and fix Registry errors.
4) Run a defrag program on your normal files. Select "consolidate free space" if that is an option. Windows defrag will do this if you run it in an Admin Command Prompt (type: defrag /? for options).
5) Fix the size of the page file and, if possible, move it to a different drive. Only move it if it is moving to a physically different drive and not a different partition sharing the same physical drive.
6) Run a defrag program again, but this time run a boot-time defrag.
If there's a slot to fit a replacement graphics card on the motherboard you could ask at a "reputable" local computer repair shop to see if they can fit a faster graphics card for you (I say "reputable" because the other type will try to talk you into buying a new or reconditioned PC). It should only take an hour or so to fit one, so they shouldn't charge more than £50 to do it, on top of the cost of the new card.
I bought a Dell desktop some years ago and, after upgrading the graphics card a couple of years later, I was annoyed to find that Dell had fitted really slow (cheap) RAM chips. I upgraded the RAM to the fastest the motherboard could handle. That bought me a few more FPS. It's an expensive way to do it though, but I thought Crysis was worth it.