The XP license on the Advent laptop will be an OEM (original equipment manufacture) if it came preinstalled with XP meaning it's applicable to the laptop only. When it was registered it takes various "electronic" tags from the hardware... so the answer there is it's not transferable.
If you have a retail copy of XP, meaning you have bought XP on a CD disc together with license and it is not an OEM... then that is transferable.
Vista is an excellent OS, so it sounds like you have other issues. If you restore from a hidden partition that brings it all back to "as new".
Do you know which update fails ?