First off, if you're going to use it on an older computer, I'd recommend against virtualization. It's too resource heavy. If you can't reformat your drive for whatever reason, go with Ubuntu and make yourself a live CD. You get all the functionality of a full install, but you can't (easily) store data.
I'd second the recommendation for Ubuntu. It's about as close to plug and play as you can get (with an OS). It's extremely easy to get into from a Windows user's point of view.
As for Gentoo, I'd recommend against it (for a new person). You'll have to do everything by hand. Installing new programs, while handled in an extremely versatile manner, is very complex. It revolves around downloading the source code for the program, then compiling it with the correct flags to give you the functionality that you want. If you miss a flag, then you lose that function. Plus, in my experience, Linux rarely crashes, but when it does, it explodes. Gentoo isn't easy to fix.
If you want a good intro to Linux, I'd recommend Arch Linux. It's very small, light on resources, but requires some learning to get running correctly. It's the best choice for people who want to learn the guts of Linux. If you go this route though, you're better off installing to your hard drive and getting rid of Windows.