Just keep your virus scan software active at all times. And set it so that it automatically installs new virus definitions.
There are also countless XP security and performance tweaks and there's a wealth of information on the web.
http://www.blackviper.com/WinXP/supertweaks.htm is a great resource. Some of the tips are pretty advanced and some are just unecessary (like running bootvis) but I cut my memory usage in half following that website. My computer runs a lot faster.
Also turn off hibernation in the power controls if you don't use a laptop. It can save quite a bit of disk space. I have my browser cache set to 128MB.
I would also uninstall all the unnecessary software that comes bundled with your computer and see if there are better alternatives to the software that you do need like WinAMP instead of Windows Media Player and Nero instead of XP's built-in CD burner. There are also Quicktime and Real Alternatives. And SpeedUp for Acrobat Reader which dramatically cuts its loading time and memory usage.
Resolution, color depth, and refresh rate settings are personal preferences. Keep them at the lowest settings that you feel comfortable looking at.