Given that this is in a thread about Microsoft Server, I'd recommend using Debian as the distribution to replace Microsoft Server.

If you want a desktop, I'd start with a LiveCD version and familiarise yourself with the various available distributions on offer.

The intent of a LiveCD is essentially to boot into Linux without modifying your hard-disk and keeping your existing OS unchanged.

I'll note that many of these images are available for DVD or USB. Some will offer a mechanism to store data on your existing drive without wiping anything.

With USB drives being fast and cheap, you can also often use a LiveCD to install onto an external drive.

Finally, you can install a virtual machine on your computer and use it to run your Linux tests.