"Holding anger is a poison. It eats you from inside. We think that hating is a weapon that attacks the person who harmed us. But hatred is a curved blade. And the harms we do, we do to ourselves."-Mitch Albom
Joking, keep your XP. But be aware, you can be at risk. So I wouldn't do anything related to payment stuff via that PC.
1 GB ram, it has to go slow for you
-How do you survive.
XP: It's light and simple, old and outdated and is no longer supported by Microsoft (Securety problems to come)
Vista: This would requir some GPU and CPU power due to the "graphics" done by the system
Win7: This is what most would say is best. It's stable and okay on resource usage
Win8: Some dislike it, some love it. It's fast to boot and such, and also fine for gaming. (It can be confusing for "newbies")