There's a free program called tex2gif (you should be able to google it) which I've never used but seems to do what you want.
If I may I'd like to offer an alternative. Have you checked out the prosper package for latex? It is a latex package designed for making slide shows. When you're done it just outputs a pdf but it is pretty cool the stuff you can do with it. It may not have all the frills of powerpoint but you can make slides appear part way at a time, or have parts change. Any time I'm doing a talk these days and I need slides I do it with prosper. It's really nice just doing everything in latex instead of mixing and mathching.
I found text2gif online, so that should be ok. The presentation's already written apart from the formulas, but I'll definitely give prosper a go next time. Got plenty more presentations to practice on. Thanks for the tips!