Ahhh. reinstalling the OS. Sounds like fun.
Actually a good idea for Windows users to do it once in a while. I reinstall XP every 3-4 years, and it's like having a brand new computer again. Honestly though, there is no painless way to do it. I don't like backup software or methods that compress files. You just need to go through everything and decide what you need to keep. That means everything. I always make myself a checklist and go through it. It seems like you've got a good idea of stuff you don't want lost so just go through it all and make sure you aren't missing anything. Here's a few off the top of my head.
-backup firefox bookmarks
-check all passwords you may have saved and won't remember when you reinstall including passwords for modems, routers etc.
-backup e-mail messages
-get all purchased software license keys (or cracked version installers and passwords

)
-any drivers for older hardware that may not have any online updates like old mp3 player, usb bridges etc.
-obviously you will be moving all files to the external so I don't need to tell you to move your wedding pics and video that you only have one copy of somewhere safe

-make a note of all codecs you may have added and freeware that you will have to get again.
After you install windows it will be like setting up a new PC. Make sure you go through all the windows updates. This can take a while because of constant rebooting. Make sure you don't go on the internet too much until you get ALL the updates for security reasons. Also setup some antivirus software before you get on the internet.
If I can think of anything else later, I'll add more. Sort of late here so almost bedtime.
