qwakers:
if its a old xp laptop it will be 32 bit.
Weather is supports 64 bit is down to what processor it has fitted, even some very old processors will support 64bit.
That said if the laptop has less then 4GB ram there’s no need for 64. If it has 1GB or even worse 512mb I would see if you can get an upgrade.
Also when you buy Windows 7 you’ll get a serial number and a disk. Although the disk will be either 32 or 64 bit the serial number can be used with either version.
EDIT: DRAM, Solid State Drive (SSD) & Memory Upgrades | Crucial UK is handy for checking if your laptop will take more memory and what type it needs.