IPB is not ASP, everything mentioned so far is PHP

You can definitely start with phpBB and later move to vB or IPB. That's actually what most forums that are not with vB or IPB did. Both software have conversion tools, so you dont need to worry about not being able to switch down the road. If you are indeed looking to start with free software first, I'm going to have to suggest yet another piece of software, haha...SMF - http://www.simplemachines.org

It's still fairly new in the forum world, but it is growing at a rapid pace, as is the popularity. Personally, I prefer SMF over phpBB...it's faster, cleaner, safer, and advancing faster than phpBB.