Nick Barrett's advice in an extract form his new family history book in the Sunday mirror said dont get a Online family tree programme until you a intermediate researcher.
What a real cracking piece of advice that isn't!!!
Using an offline tree builder means you have complete control of your tree and all the information that goes with it. Anyone just starting out will never appreciate just how much there is to keep track of, bmd refs, census refs and information etc. names, dates, places.
Relying on an online tree buiilder presents problems, not least is that you gave to have internet access for a start, sites can and do suffer server problems, and they can never gaurantee that your info is secure. Creating back up copies can also be a problem.
Putting all the bmd refs, names, dates, places, census return info, sources, certificates, transcriptions of wills etc onto a family tree programme early in your research is far better and also far easier (there will be less to input as you have less information available). Trying to enter information for say 200 individuals at once is quite a task, far easier to start with a programe when your tree is far smaller.
For the cost of a programme (as little as £3 if you know where to look) it's hardly worth waiting.
As for a programme compatable with XP and Vista you could always try Family Tree Maker (either the 2005 or 2006 version). To work with Vista you need to download an update patch but if one pc works on one system and one on another then you simply install the software to each computer and uploasd the patch to the machine with Vista.
FTM 2008 isn't that popular with users, i know several folks who upgraded from an earlier version and still use the older programmes in preference to the newer one.