First, if your XP is an OEM version, then it's not transferable to the new pc.
If it's a retail version then you can reuse it, however trying to get it boot off the old drive is problamatical.
You would need to discconnect the W7 drive before you start messing withthe xp drive.
If it fails to boot you can try booting off your xp disk and running a repair install, if you are succesfull then you will need to install the drivers for your new hardware.
A clean install of XP would be a better option.
You can now decide if you want to create a dual boot menu, or keep the 2 OS's totally independant and select the boot drive from bios.
Dual Boot Installation with Windows 7 and XP.http://www.sevenforums.com/tutorials/8057-dual-boot-installation-windows-7-xp.html
Only use one firewall.