When installing software in linux that's not a binary, you get the actual source code, the actual files that are used to create the binary. the standard routine for installing software is using tar to extract the archive, in this case tar xvf(z|j) gtk....tar.(gz|bz2) (z or j depends on which you download, the .gz or the .bz2
then, you cd (change directory) into the newfolder. (Usually the exact same as the file name without the extension.) then, after reading the README, type ./configure
this will search your system for the things it needs. Once this is done, or it tells you what you need (which you then scurry off to fetch, and install), type in make. Make will go through building everything it needs. All of this stuff can be done as a normal user. then, after a compile process, become root (using su), and type in make install. This will install all the files and folders.
And tada, you have manually installed your first package.
Now, I really can't stress how much you should probably upgrade. RH7.2 is ancient, and you will find that many, if not most, modern packages will barf on trying to install because of the oldness of your system.
Your box may seem old in the windows world, but it's more than fast enough for linux. I'd reccomend trying a debian based distro, as it has the strongest and most newbie friendly packagmanager, called apt. (advanced package tool). Apt will download and install everything you need to run a piece of software.
A nice debian-based distro is new to the scene, called Ubuntu. They will even ship you a CD if you want, from here
http://shipit.ubuntulinux.org/ http://www.ubuntulinux.org/
Now, if at first it seems slow, it's probably because it's running Gnome 2.8 by default, which is a fairly heavy desktop. So, one of the first orders of business will be to either switch desktops, or try and get it runnig faster (by slimming down the effects, etc.)
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