Well right now the best thing you can do for your computer is browse through the downloads section on SysChat that is filled with gems that can keep your new computer functioning properly (and secure). Other than that you might want to run auto update, and get all the latest patches from Microsoft and so on. You might also want to familiarize yourself with all the XP functions (since I think you were previously using Win 98 ).
Then make sure that you defragment your computer every single month, and use the Disk Clean up utility to keep it running swiftly. You might want to use task scheduler to do these tasks as well... And make sure that you update all your drivers as well, so that all your hardware is going to be running perfectly. You can also take a look here for some tips with people with new computer...
Oh yeah and one more thing (hmm... Steve Jobs' keynotes are contagious ) if you've bought your PC recently then you might want to take a look at this offer from Microsoft... , lol, I was just kidding about the last one...