I'm wondering if I could use the MS Office install discs that came with my Windows based computer on my new Mac...it MS 2003 version for XP.
Well, as had been said, no - but aside from that, if you're splashing all that cash on an ultra-slick new computer it'd be a
waste to continue to use Office 2003 anyway! There're much better productivity suites out now to get the most out of the capabilities of your MBP:
For convenience, you can get your Mac pre-installed with Microsoft Office:mac 2011 (more familiar for switchers, but fairly expensive at GB£89.95 / US$119.95) and/or Apple's iWork 2009 (more Mac-native, office compatible, and quite a bit cheaper at GB£57.00 / US$49.00).
You will notice that iWork, though still the cheaper option, costs way more in Britain than in America - closing the gap between it & Office:mac - so your choice of which suite to go with may depend on where you are in the world.
are there any free or low-cost document converters out there?
As said by
Buzzard2010, if you want value, go with OpenOffice or LibreOffice; both free, both Office (& by extension iWork) compatible, & both comparable to iWork and Office:mac in general usability, just less fancy looking!
Whichever way you decide to go, good luck & happy computing