(and reformatting, as thats the next logical step)
Backup Personal Files
First thing you should do is turn off any encryption on your files. On any other computer, the files are gibberish so to protect them. The thing is, when you reformat your computer, it becomes a new computer, making all your files unrecoverable. Hopefully you remember which files/folders you right clicked and turned on encryption for. Otherwise, you should be aware that Outlook stores your mail encrypted, but how to backup that is for some other guide, as i don't condone the use of outlook.
The most obvious thing to backup is all your files in 'My Documents', the Desktop, or any other folders you have created to stored personl files. Where do you save your:
Work?
Pictures?
Messages?
Documents?
eMail?
Music?
Videos?
Downloads? (especially if you have a slow connection or the program is no longer available)
Important Miscilaneous Data
All Your Passwords, Usernames, Product Keys, or other 'saved' form data
Bookmarks
Saved FTP, printer, or network place addresses
The windows address book if you use it
Scheduled tasks
Save a copy of the entire Registry (run->regedit->click My Computer->File->Export)
Backup Program Data, Files, and Settings
The next and possibly longest procedure is to go through every program/game/utility you use and backup files you can think of. These files and settings can usually be found somewhere in the folder where the program is installed (in program files), My Documents, Application Data (documents and settings/username), or in the Registry. Go through each one and ask yourself:
Do i have the original install CD or download (and activation keys)
Where do i save this program's type of files?
What extensions or updates did i install for it?
What skins or plugins did i install?
Do i remember my username/password or other 'saved' form info?
did i make a lot of settings changes that i want to backup?
Is there data saved in the program that i want to extract?
Locations of Windows Files
Wallpapers C:\Windows\Web\Wallpaper
XP themes C:\Windows\Resources
Cursors C:\WINDOWS\Cursors
Sounds C:\WINDOWS\Media
The Desktop C:\Documents and Settings\USERNAME\Desktop
My List of Program-Specific Backups
Firefox:
plug-in list
bookmarks
skins
Aim:
Away messages (registry)
profile (application data)
username/password
MSN messenger:
MSN Plus!
Stuffit Plugin
Winamp:
Skins
Plugins
playlists
Bookmarks
media library
Media Player Classic:
config file
ObjectDock:
themes
backgrounds
icons
CursorXP
cursors
Free Launch Bar:
folders with settings
Games:
game saves
main executable
downloaded updates, extensions, extra maps
Google Earth:
bookmarks
AVG
username and password
Matlab programs
Stepmania
Joystick 2 Mouse
Electric Sheep Saver
Reformat Procedure
Install Windows
Update to Windows SP2
Do windows update (Restart and repeat)
Setup firewall, antivirus, and antispyware
Download and install the latest drivers
Install DirectX
Install Codecs and make other Windows modifications
Start installing all your programs and Copying back your files
dont forget:
          Java
          Shockwave and Flash
          Quicktime and Real Alternative