Hi Jim
Mac (and PC) users have found the Keyspan USA-19HS to be very reliable. Obviously, being a "for Mac" product you'll have to pay an unreasonable price for it. Any USB to SERIAL adapter costs a fraction of that and works fine. Yours is the choice:
http://www.tripplite.com/en/products/model.cfm?txtSeriesID=140&EID=14871&txtModelID=3914
Regarding which XP version/licence you have to use with Parallels, please check their website for specific requirements.
good luck
Italo