might answer a few more questions for you; from there (if you have a clearer picture of what you want), go to the various manufacturer websites and see what they have to offer.
As for PC security programs; there are alot to choose from, anything from the most basic to programs for intermediate PC users. It depends whether or not you trust using free programs or would rather spend money for a program.
Buying a desktop. Want a system that will handle music, video, web (including some with heavy cpu usage like Google Earth) with speed, but don't do any gaming. Is a Pentium outdated and best to stay away from? What about DD2 v. DDR3 RAM? Thinking of buying a Dell, but have looked at HP, as well.
Also, want to prevent malware/viruses from making my new system slow. What is the best way to go about it (i.e., firewall that comes with the system? Specific anti-virus software? Can I just stay away from untrusted sites?).