I would highly recommend backing up your data and re-formating your drive once you pop it in the new rig. As the others have said, XP will usually throw a tantrum if you just swap the HDD into a new system like that.
It can be possible to get around it at times, depends on what the old and new systems were...but it's more trouble than it's worth I think.
On the plus side, at least you'll have a fresh XP install after doing this, which is always nice.
If you are transferring PATA to PATA, most likely you will not have many problems. If you are transferring SATA to SATA, you need to replace the SATA drivers with generic ones.
I recommend uninstalling all drivers motherboard specific or card specific prior to making the transfer.