That 20 inch iMac? It's not an Intel machine... That would probably explain a lot of the issues. It's an old IBM PowerPC machine, 2005 at best. And 10.5.8 is the newest OS it will ever run, as 10.6 dropped PowerPC support.

Print Screen functions do exist in OS X, but it's not a single key (by default, but can be reassigned). http://osxdaily.com/2010/05/13/print-screen-mac/ shows the various keystrokes, which I'll admit, are arcane and took me a while to remember.

FYI, Mark, that system likely included a 4 button rodent. Apple hasn't shipped a single button mouse for 8 years. Apple also has supported multi button mice for OS functions for nearly 13 years, with 3rd party apps making use of them even longer. Perhaps upgrading from Lynx will help show you the more modern web with this news (that is still the default browser on your OS right?