You can buy a non-registered copy of Windows. It will cost about £2,000 but you can install it on as many computers as you want.
Also I'd highly like it if we stop discussing the finer points of copying Windows XP and forging activation codes or getting around them. Did your other computer not come with Windows XP?
Major updates to the hardware of the computer can be allowed by Microsoft if you phone them up and describe the nature of the upgrade.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.