I can't help you STEP BY STEP as I use Outlook 2007 but I'm sure it's almost the same.
Goto EXPORT and read the step by step thing and just choose .pst and then then when you reinstall, import that .pst file and you'll be fine.
To save your adress book go to (in outlook express) Tools->Adress Book , Then select File -> export adress book (WAB) and save it somewhere save.
The above will only save your email contacts.
Email messages are typically kepty in a directory such as
C:\Documents and Settings\Home\Local Settings\Application Data\Identities\{09C9ED6D-4FB7-47BB-9C97-07E4293C629F}\Microsoft\Outlook Express\
Where Home above is my pc identidy yours will probably be different, generally searching for the outlook express folder in windows will show you the path. Save this whole folder,
and then import it later after you have formatted.