I had to do the same thing last week after a succesful Exch2k3 upgrade, then we upgraded the OS from 2k to 2k3.
It died and nothing would fix it.
As only the OS was screwed I just reset the Exchange server AD account, reinstalled the OS joined it to the domain as the same name server it used the existing machine account.
Reset the drive letters so they matched the old installation then re-installed Exchabge 2k3 with the /DisasterRecovery switch.
Updated to Exch2k3 SP2 and remounted the stores instant recovery.
All working sweet as.
So if you have an offline backup it's as simple as 1,2,3
Henrik Walther published a walkthrough here...
http://www.msexchange.org/tutorials/...ry-Switch.html
I have used it several times without problems.
My golden rules for exchange,
NEVER install it on a DC
Always have it on a partion with twice as much space as you thing you will allow the stores to grow to.
Always do an offline backup before performing any major changes to the server or the OS