So Outlook 2003 and 2007 store the email account details there.

Which you need to know BEFORE you rebuild your PC.  (So you can take a backup copy of it).

Not that obvious until you need to know where to find it:
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\Outlook\9375CFF0413111d3B88A00104B2A6676