As above -
If your copy of Windows XP is legal, you own that copy in perpetuity (forever). As long as it is on one machine, you can use it, move it and update it legally.
Also as above, you may need to reauthenticate/reactivate the software with MS if you change machines or hardware, otherwise you can experience problems (updates, operation, etc.).
The above applies to OEM/retail versions of their software, whether preloaded or not. Other license schemes, business versions, etc. may be different.
Dell