There are several ways you remove the paint but whatever you do if you are planning on using a concrete acid stain don't use muriatic acid to clean the floor. Franmar.com carries a product called SoyGel that is excellent for removing paint. It is manufactured with soy bean esters and is therefore both eco-friendly and safe to use.
The second method would be to pick up some Citrus Strip from Home Depot or Lowe's, it is a citrus based product and safe to use.
The third method would be to scrub the flooor with a black pad and a floor buffer and it might be necessary to put a sandscreen under the buffer pad. A sandscreen is similar to what they use to sand drywall with but it comes in 17" round screens that won't scratch the concrete surface.