Virge is correct. You have to read the stem question - OP wants to put Win 7 Upgrade on a new machine using the old machine's XP license as a basis for the upgrade.
If the old license is OEM, You might get away with it if you first install the old XP system on the new machine. In any case, Win 7 must be a clean install when upgrading from XP.
Larry suggests upgrading the old machine to Win7, and then transferring that to the new machine. Sounds good, but again, it has to be a clean install on the new machine because simply transferring an OS to a different machine usually results in a BSOD on boot.
If the old XP is full retail, no problem at all. Install 7 on the new machine and use your old XP disk to qualify.
And, you can always call Microsoft with your old key and see if they will give you one that works.