Older iMac not loading users properly

[Anicura]

I have an older iMac (I'd say it's at least 4 years old) that's still running Tiger on it. Recently, it's been getting progressively slower and slower every day. A few days ago, it wouldn't force quit World of Warcraft, so I forced the computer to shut down via the button on the back (was in full screen mode, computer was not usable whatsoever because of this). When I turned my iMac back on, it makes the start up noise, the gear animation follows, then it loads a blue screen (same color as where users should show up). It doesn't load any users, and occasionally it just opens a Terminal screen that takes up my entire screen. Currently, my computer isn't usable at all. I'm not very tech savvy, I've been told I need to reinstall my operating system (I don't have my old Tiger discs either), and I'm wondering if you guys have any advice. I'm thinking I might have to buy a copy of Snow Leopard and see if it'll work, but after trying to run a Linux disc in the computer and not getting a response (it makes regular disc loading noises, but that's it), I'm not entirely eager to cave in and spend $35 on software that may not work. Any help would be appreciated.