If you will not "keep" anything from the first computer and the XP is not an
OEM version,you just install it on the new computer.
If the version of XP is an OEM one, you will need to buy another retail
version. OEM versions can not be transferred to another computer, as part
of the OEM EULA. Also, some OEM version are locked and coan only be
installed on the original computer.
