Windows installation customizes itself to the hardware it detects, selecting drivers for the mobo chipsets, graphics and sound cards, monitor etc.
It's extremely unlikely that XP pulled from one PC will boot on another unless the two are virtually identical in make and age.
You can use EasyBCD to add an XP entry to the BCD after you plug in the drive. It will detect XP and do everything you need. Just let it auto-configure and don't change anything it sets up.
That's the easy bit.
However - when you call XP from your boot menu, expect it to fail immediately.
If you have a full retail version of XP with a MS installation CD, at that stage you can carry out a "repair install" of XP which will tailor the OS to the hardware on which it now finds itself.
If however, your old PC was purchased with XP already installed, then it will be an OEM version. The licence doesn't belong to you, the owner of the PC, it belongs to the hardware on which it resided when you bought it, and if the h/w dies, so does the validity of the licence under the terms of the EULA.
That copy of XP doesn't contain a universal driver set, just those of the specific h/w it came with.
Even if you borrow a copy of a full retail CD, your old PC's serial number won't allow re-installation on a different PC. (That is to say, you will be unable to activate the key and MS phone support will decline any request to do so)