First of all, you should try to configure your modem (make sure it works under Linux) before you think about getting an ISP - and if you already have one under Windows that you're happy with, why change?
Mandy 9.1 comes with a little WinModem config tool - but it is by no means perfect, and many if not most WinModems (by their very nature) will not work with Linux. You could also check out linmodems.org (google for it, if I've spelt it wrong).
Oh, and you're asking for free ISPs, but as of yet, we don't know where you're based. An ISP I suggest, in the UK, will probably be of little use to you if you're a USA resident, or if you live in India... just a thought... fill out your user profile.