If you could upload a screenshot of what it looks like, it might be easier to see what the problem is.
To take one:
Start playing
Press PrtScrn (or similar) on your keyboard when you want to take the screenshot
Exit Halo
Go to Start-->Programs-->Accessories-->Paint (or your paint program)
Click Edit, then Paste (As new image in some cases)
One thought though, have you changed any video settings on your computer or on Halo recently? Perhaps you've made some change to your settings that the video card can't handle? Perhaps try fiddling around with a few of those.
Short of that, I have no idea (and I don't see how a reinstall would make the problem worse)