Does anyone know of a great way of removing spray paint without doing to much harm to the original paint?

Lacquer thinner?

and it will not damage the original paint?

Never has for me. As always, try in a small, inconspicuous spot first.

Are we talking spray painted, or just some over spray?

I bought a bike that was stolen from the orig owner and rattle canned red, she got it back but it has a butt ugly layer of red paint all over it.

If you knew what kind of spray paint is on there and what kind of paint is underneath it, you could might be able to figure something out. Otherwise, you'll just have to try solvents or maybe light wet sanding. If you don't like it the way it is, you don't have much to lose.

Because of the difference in the two paints, any solvent you use that effectively removes the spray paint should turn it kind of gummy and peel-able--even if it'll damage the original powdercoat, I'll bet it'd take far longer for it to take effect. If you work on a small section at at time, applying solvent, then peeling away the spray paint, you should be able to wash/wipe away excess solvent before it causes too much damage. Tedious and time-consuming, but effective.

Oh, and I'm sure it'll help if everything's a bit warmer--maybe set the bike out in the sun for a while before starting.

As others have said, try a solvent. If they didn't use primer, then there is a good chance 90% alcohol, or mineral spirits may help in the removal of the paint. I would also bake the frame before trying a solvent. Put it in a black plastic bag, and set it outside on a sunny day, come back in 10-30 minutes, and give the solvent a try. Careful, the frame may be hot (~180 degrees F on a sunny warm day).

Cool, thanks to all who replied.