When you uninstall Outlook it never removes your e-mails (unless you ask it too). They should all be located in the C:\Documents and Settings\HP_Administrator\Local Settings\Application Data\Identities\{D190EE07-1887-4595-8F62-6253314288D2}\Microsoft\Outlook Express
The HP_Administrator will be whatever your bosses login name is. So if he just uses administrator then replace the HP_admin~ with Administrator.
Also the crazy letters and numbers {D190EE07-1887-459... will be diffrent on your computer also. So your best bet in finding his outlook folder is to just manly navigate to it.
Open explorer go to c:\Documents and Settings\ click on the folder with his username. Go through these folders.. Local Settings\Application Data\Identities\ then click on one of the folders with weird numbers (theirs probably allot since you guys installed outlook a few times) but you can make sure what one is the real one by viewing the folder's file size. You said he had allot of e-mails marked back in 2001 so this means his folder is probably going to be huge. Probably in the gig range. So check all folders and find the one with the biggest number. Inside you will see .dbx files. These (I think) can be imported into firebird. If not then just install outlook again and move these files into the new outlook folder and you should see all the e-mail show up in the program next time you load it.
Hope this helps.