I've seen it on plenty of 97-99 style headlights, and it's been bugging the hell out of me. It looks like dust and crap, but it's actually fine rock chips on the surface of the lense. After a couple tries I found a method that works for getting rid them..

You need grits 320,600,800, and 1500 of Wetordry sandpaper, Mother's metal polish, a rag, Plastic polish, and a buffing wheel (I used a plug-in Black&Decker 7" 2 speed, 3/4 HP buffer).

Start off by removing the entire headlight housing from the car. Begin wetsanding the surface with the 320 grit and KEEP IT WET. Take your time and make sure you sand equally over all of the lense. Keep at it with the 320 until the rockchips are gone. Next, move on to wetsanding with the 600 grit, then to 800, and 1500. I took at least 7-10 minutes with each grit for best results. Now, the lense should look really fogged up and way worse than when you started. This is normal!

Take a cotton rag and use it to rub metal polish across the lense. Begin buffing it by hand using the rag. Be generous with the metal polish and put some muscle into it. Take at least 10 minutes with this stage.

Now the lense should look clear, but no very glossy. Take another rag and use it to smooth plastic polish over the surface of the lense. Now use the powered buffing wheel to buff the lense until all the polish is gone and the lense is looking glossier. With an amazing 3/4 Horsepower-powered wheel, I had to secure the headlight housing between my feet. Do this 3 or 4 times.

If you completed all of the above the headlight lense should look as good as new. Stick the housing back in, switch on your lights, and be amazed.. It looks clean yo!
Here's a before/after comparison shot I took. The difference is day and night in person, and now I have no reason to burn $300 on projectors.