As Amanda has mentioned, Linux Mint is a great FREE OS. There is also a free antivirus for it by Comodo (there are people who say you don't need an AV program with Linux, but I still like one). Linux Mint is very similar to Windows, and there are two "flavors" that run very well on older, less powerful computers. You can always just take a look and see what you think.
I have a computer that I built in 2005 that has Linux Mint 17.3 MATE installed on it and it works perfectly.
Again, it's just an idea. It's good that you have Windows 7 on your laptop so you are able to do more financially sensitive things that you wouldn't want to do on a vulnerable computer.
Installing it is fairly easy, and using it is a breeze. There is a wealth of information on the Linux Mint site, and on many other sites on the web on how to install and use the OS, as well as updating things and such. Also, which version and "flavor" would be best to use for your particular machine. You basically download it and write it to a DVD or flash drive (older computers may not be able to boot with a flash drive) and install it from such. Again, though, there is a lot of info on it on the site and around the web.
If you can, I would use your Windows 7 laptop for now, especially, again, for anything more sensitive (paying bills and shopping and such). Although Windows 7 is out of the mainstream support cycle, it's still receiving critical security updates, so that's good.
Do you know what processor it has and how much memory? I can't remember exactly how to find out on XP, but I think you probably right click on the My Computer desktop icon and click properties. I think it will show you there what processor and how much memory you have.