that your laptop is under-powered in the RAM department, and that accounts for the slowness part of the problem.
As for "loads of stuff I don't want or need", that's a puzzle - do you mean Vista components, or other installed software? If it's the latter, just uninstall the stuff.
"Can I load XP onto it as an upgrade?"
"can I save it onto a hard drive/disks and reload it at a later date somewhere if I want to?"
This is an old chestnut, and the short answer is yes. In the past Microsoft confirmed to me that it is quite in order for a consumer to make a back-up copy of the Windows system files that are stored on a separate partition on a new machine. You may then use this backup to install the operating system on a different machine, provided the original installation is removed - in other words you can only run one copy of the software.
"how do I do it?"
You locate the partition on which the files are stored, and write them to a DVD. Make sure you keep a careful note of the activation key that came with your new laptop, lose that and you can't use the software. The key will be on the printed Microsoft licence label, and on a laptop that is usually stuck on the underside of the case.