If you're concerned about installing SP2, make a backup image of your C: drive first. If everything goes disastrously wrong, you can quickly put everything back the way it was before.
You can use the 30 trial of Drive Snapshot and store the image on your external drive. It's always a good idea to have a recent backup image anyway, so you never have to install everything again from scratch.
Drive Snapshot can be downloaded here, or you can use Norton Ghost or Acronis TrueImage.
http://www.drivesnapshot.de/en/index.htm