It's time to have a look at what is running behind the scenes.  Look in the systray, the space down by the clock.  If you have a bunch of icons in there, they are running all the time and hogging up computer resources.  A good example is find fast a utility that comes with Microsoft Office.  Every two hours, find fast indexes your hard drives to find your microsoft documents.  It causes your hard drive(s) to grind and causes really nasty lag.

1    look in your startup folder and take out whatever you don't need.  Right click on the start button and select explore. Then go to the programs folder, then the startup folder.  Remove anything you don't think you need to be running all the time.  You can just drag those shortcuts to the desktop so you can still use them when you like.

2    hit the ctrl, alt, and delete buttons at the same time.  Be careful to only hit them once, twice will reboot your computer.  You should see a window that looks like this.....

These are all the programs running on your computer right now.  Each time you want to play Tribes, close everything except   Explorer, Systray, and Rnaapp.

You may have a program running that does settings for your video card in the window.  You don't need to close that one either, but it can be closed.
 

Getting closer - let's keep going