Venae, pulsium verum ◦ 暢気
Are you trying to use a potato to play FGunZ?
Even with the specifications you stated above, your computer system should be able to run FGunZ. So I'm assuming your PC is so 'clogged' up you might benefit from performing some of the following actions listed in this article.
http://www.wikihow.com/Speed-up-a-Windows-XP-Computer
You may be familiar with some, but you not be with others.
Other than that, I strongly recommend upgrading the components of your PC or replacing it entirely with a new retail one.