How much time are you willing to put into this?
I would probably do the following:
- Get a new reasonably small hard drive - SSD - for about $70
- Back up all of your files and documents from your old hard drive onto a usb stick
- Replace hard drives, install linux (ubuntu, or you might prefer linux mint) onto the new hard drive, drag all your files over
- If you ever need old stuff, simply swap the drives, OR buy an external hard drive enclosure for the old hard drive and plug it in when necessary
That's probably the best way. An alternative would be to install linux side by side in a dual boot configuration, and use linux instead of windows, but whenever you need your old windows stuff, just reboot into windows (preferably without internet...)
By the way, on such an old machine, I would much rather put linux mint than ubuntu. It's much lighter on your resources (cpu and graphics). Otherwise it's basically the same thing, so everything that works on ubuntu works on mint. As a plus side, mint resembles XP much more than ubuntu does. In either case, though, you'd be able to pick up how to use it within about 15 minutes, it's all straightforward - mouse, keyboard, click on programs to run them, you get the point.