Well as g-paw said, there are 2 ways to go about it.
1) A clean install is best, gives you 100% chance of stability etc etc. Also great for cleaning out all the crap that accumulates over time.
2)You can try just sticking in the hard disk to the new pc. However, windows will hang, as it won't recognise the new hardware, especially the storage controller if you have it set to RAID or AHCI mode.
The easiest way to fix:
1)Connect up your hard disk to the first hdd port on your PC (Master if IDE)
2)Set you PC to boot from CD-ROM in the BIOS
3)Load up the window setup
4)It will complain that a version of windows was detected etc etc. Select repair
5)Reboot after setup is complete
6)Once windows starts, it will re-detect most of the hardware - the repair resets all drivers to default/removes them.
NB - the repair can also reset some programs such as IE7, WMP11 and so on, that don;t come default with XP SP2.
Hope thay helps you.