Awesome question

I think I can help you out a little, before I sign off.
Virtual memory=physical RAM+
swap file(AKA paging file)
Windows has a
swap file that is a portion of your
hard drive, but used as RAM. It is not as fast as RAM, but it is better then not being loaded at all.
If you look at your desktop (the physical one that you can knock, not the one on your computer screen), then your RAM is everything withen your arms length: your keyboard, mouse, paperwork, and even monitor are some of these items you can touch.
Using this, a swap file is like a shelf behind your back, withen arms length. You can still reach it, but it will take longer to move your arms, get the items, and place it in front of you.
When your desktop becomes cluttered, you want to move some of the items back to the shelf behind you.
This is very similar to have a swap file works. Windows knows what has been used recently, and what has not been used in a while. The applications and files that have not been used in a while, are placed on the "shelf", and loads it when needed.
Knowing this, there are a couple of things you can do to get Halo to run better: add more RAM, eliminate running programs, or get (upgrade if you already have one) a video card.
Because adding more RAM or adding a video card would not only cost money, but would require a little more research catered to your specific computer, I would like to offer a few pointers on playing games. Upon my return later, I would like to explore the option of those though.
-If the game does not need to be online, unplug the internet. Then, and ONLY then, you can disable your A/V program and firewall. Be SURE to enable those programs
before you plug the internet back in.
-Defrag your hard drive in Safe Mode. This will give the computer a larger space to work with. While you are there, you can even remove some applications in the add/remove programs, and delete your recycle bin. If you do these extra steps, do so before defragging.
-Do not have any other applications running. Close as many as possible, but
DO NOT use the Task Manager, Processes tab to do so. These are not the same as applications-once again
LEAVE THESE ALONE. If you want to use the Task Manager to close applications, then do so in the APPLICATIONS tab.