First, installing an operating system is a major issue on any computer. If
you are starting out by worring about what you might lose, it means you're
not prepared and probably shouldn't make the move. Being prepared means
being completely backed up and able to reinstall your previous setup, all
applications and restore your data from scratch.
There simply are no guarantees. A good place to begin as was suggested is
the Upgrade Advisor:
http://www.microsoft.com/windowsxp/pro/howtobuy/upgrading/advisor.asp
Still, you should be sure you have all your application setup disks, you
should check the application web sites for compatibility with XP, if patches
are necessary or if they have updated versions and be prepared to purchase
them in the event your current versions have problems with XP. The same is
true for your hardware. Even though XP has quite a bit of backward
compatibility built into it, you must understand, most of your hardware and
software was not designed with XP in mind.