Well, to cut a long story short - received an update from Vet Antivirus. It was not specified that SP6a was required and as a result this particular server Blue Screened and was unrecoverable.Restored from tape the OS partition (Exchange files on another partition).Rebooted, had a couple of problems starting the IS, but fixed with isinteg.
Recently installed SP6a, rebooted IS wouldn't start - database inconsistent.Ran all isinteg checks - no luck.Ended up backing up priv.edb & pub.edb then ran eseutil /p /d followed by isinteg -patch -fix alltests a few times until there were no more warnings/errors. IS is now stable, but all attachments have disappeared and my CEO wants to string me up.....
Tried creating another EXCH server(on another machine) but IS will not start with original EDB's. Error 1276, not the same server....