If it's running XP, it's probably running on really old hardware. Upgrading the operating system won't do it any help. I believe it's time to get a new computer. You could upgrade by adding more ram. But it's hardly worth it and I don't suggest it since it'll be bottle necked by old hardware. The more recent processors won't fit into your old motherboard socket.
The best you can do now has already been mentioned. Clean your registry, defragment your hard drive, run a malwarebyte scan, and keep your drivers up to date.
You can buy a new ram for your xp. Then it wouldn't slow down...