As FormerRushFan mentioned, it comes on a live CD that you can boot to test-drive Linux without actually installing it.
One nice feature of Ubuntu is that it can non-destructively shrink an existing Windows partition to make room for the Linux partition. I've done it a couple times now, and have had no problems, though it is rather major surgery, so I recommend backing up anything important that you might have on your hard drive.
1.)I know that Linux comes in several different versions(distributions). Are the distributions upgradable? I purchased one distribution of Linux (PC Linux OS Preview 81a) awhile ago but I've never installed it. I ran it once on my laptop off the CD but I didn't want to mess anything up so I didn't try installing it. Do you pretty much install a specific distribution and develop it as you go or are updates periodically released (like with Windows)?
There are dozens of Linux distributions, though the major ones are Red Hat, Fedora, SUSE, Debian, Ubuntu, Slackware, etc. Like I said, I recommend Ubuntu. It most certainly is upgradable - Ubuntu uses Debian's packaging system which makes upgrades and software installation very, very easy. It can upgrade over the network.
2.)Does running/maintaining Linux require advanced computer programming skills or can it be utilized more or less as easily as Windows? Although I have become rather proficient in Windows and being able to troubleshoot most problems over the years, I have yet to learn even basic computer programming and would not want to use something considerably more complicated than Windows on a regular basis?
I will be honest here - Linux systems do have a significant learning curve. It's far easier than it once was - Ubuntu and every other major modern distribution comes with a GUI that works very much like Windows or Mac OS. But like any computer, you will have to do some learning.
3.)How do you install Linux in a way that would allow me to keep my existing OS? I've heard references to being able to install it in a way so as to allow "dual-booting". How is this set up, particularly without trashing my current OS?
Like I said, Ubuntu's installer has a feature than can non-destructively shrink your Windows partition to make room for Ubuntu's partitions. Make sure you run scandisk and defrag on your Windows partition first, and BACK UP ANYTHING IMPORTANT! I've done it twice now with no problems, but this is major surgery, so take precautions.
4.)What kind of system requirements do most Linux distributions GENERALLY require to run?
Linux is very forgiving with regards to system requirements. I'd say for the current version of Ubuntu, just have a Pentium-class machine (say 300MHz or faster processor) with 512MB RAM and 20GB of hard disk space, and it'll run great! I've seen it run on far, far slower and older machines. If you can run Windows XP without too much struggling, you can run Linux easily.