very simple - but also very complicated
Before you insert the CD, check Control Panel/System and take a note of all your installed hardware (especially the modem!!!!)
Then Control Panel/Add-Remove Programs and note all your installed programs
The Reason for this is some of your hardware/software won't work with XP initially without new drivers/versions/upgrades.
Before you use the XP CD, check for and download a new modem Driver. Without this you cannot upgrade to XP as nothing else can be updated and you cannot connect to the net to download anymore!!! Copy this driver to a CD/Floppy for use later.
NOW you can use the XP CD.
Once installed XP will search for hardware and try to install the drivers for them. Click Cancel on everything to begin with, until you get the green field screen.
Install your modem drivers, then check Control Panel/System and install everything marked with a ?
Reinstall all your software as per your add/remove programs list.
Some may need updating, but that will be evident at install.