Since you didnt mention age of the stairs first thing to do is confirm there is no lead in the paint. If it does contain lead you need to call a pro to remove it. Removing lead based paint is not something to mess around with.

If theres no lead then you can use a chemical stripper but that might still leave some residue. Since you are wanting to finish the treads sanding is required. To deal with the spindles will requirea sharp and i mean sharp scraper blade. If you are able to remove the spindels i suggest that gets done. Job will go much better if you dont have to scrape around spindels on every tread.