You can find some precautions to make in this post .
A restore point will be created automatically during installation, and you will be asked if you want to backup the old files to make SP1 uninstallable. Take that option, you can get rid of these files later if everything works fine. In most cases everything works fine btw.
Last point if possible get the full "network"install and don't do the express install. You can then make a slipstream cd which will be usefull if you use SFC, add Windows components or reinstall.