I normally just take a screenshot of my taskmanager when I don't have any programs running. This gives me a general idea of what is running on the system. I have seen one or two viruses that register on the task manager when they start using resources like RAM and CPU. When I think my system is infected, I take another screenshot of my task manager without any programs running, and compare the two. If I find something that is running on one, that isn't on the original, I take a closer look at that. Most times, you can right click on a process and it shows a link that gives you some details on the process. I also take screenshots when I install or uninstall new software so that I have an updated version.
It seems a bit anal

but I've lost so many systems because of viruses, its like a pet peeve of mine.