I have a laptop (an HP using Windows XP) which ran out of its small memory. I have connected a very large USB memory and now need to boot from that. How do I go about changing the boot instruction, please?

If its an older laptop, it may not be able to boot from an external USB drive. In addition, you would need to set up the USB drive with Windows on it. Finally, this will cause it to boot slower.

My recommendation is to clean the internal drive. Move all your data to the external. Clean up the temp files and Restore points etc. Make sure any new data files are saved to the external.

You can make a bootable windows usb drive with a program called MagicISO (for linux I would use unetbootin). It's free to try. ScottGem is probably correct in that your PC may not support usb booting. Usually to get to the boot options you would need to press a Function key during POST. Usually it's F12 (at least on Dell's) not sure about HP though. You can then select if you want to boot to CD/DVD, HDD, USB (if supported), or network (if supported). Hope that helps.