Although I have no observable malfunctions of my system, I am wondering what I ought to do to make sure that I have not acquired any viruses, keyloggers, or possibly any e-mail viruses that I might spread to others, (particularly anything that might do damage to friends who still use Window's stuff).

My System is
OS: Ubuntu 8.04
Kernel:2.6.24-16
Gnome:2.22.1
Dell Studio 1535
Browser: Firefox/3.0b5
E-mail: Evolution 2.22.1

I am pretty much a neophyte at Linux

One truly basic/fundamental measure you can take is use a HIDS, such as AIDE. You should create the initial baseline right after a fresh install from trusted media and before connecting to any network. While this is a passive (and some might even say old school) way of detecting unauthorized activity, it's still very effective. If you want to go for something more advanced, maybe look into solutions like Samhain. Tools such as Rootkit Hunter are a really good idea to have in your arsenal too. If you're running Ubuntu, it might be beneficial for you to become acquainted with the AppArmor mandatory access control solution it includes, as that can prove to be an extremely useful defense mechanism from certain types of attacks. Using a virtual machine (such as VirtualBox, for example) to keep high-risk activity isolated from the rest of your system is also something I would highly recommend as part of a risk-reduction endeavor. As for limiting the spread of Windows viruses, ClamAV is a free virus scanner which is commonly used on GNU/Linux for that purpose.

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8.04 is getting a bit aged now; it is still in support - being an LTS version it has support up until the fourth month of 2011, so that is still OK, but you have, eg, Firefox/3.0b5, which is also quite aged (firefox is currently on 3.6.something, IIRC).

What this means is that to keep getting security fixes, you will have to make a move shortly, as support for the version that you have disappears; you should (mentally at least) form a plan for this. Two potential plans that makes sense might be to go for the latest version (eg, 11.04, in the April), which seems to imply that you'll be updating quite frequently. the other obvious option would be to stick to LTS versions, and that would imply changing to 10.04...which you might as well do now. And, the disadvantage with the only-LTS plan is that you end up with quite old apps towards the end of its life cycle.

Which is preferable depends on your priorities, but don't just let support elapse and therefore have a system without the availability of security fixes.