Iit cannot find the backup. how do I get it to use the backup I know is there?

try this command in the command line:

wbadmin start recovery -version:03/05/2008-01:31 -itemtype:volume -items:c: -backuptarget: -recoverytarget:c:

NOTE you need to pull the version from the command: wbadmin get versions

Thanks

but not sure what this means

ok so you just
1. go to Start > Run > type "cmd" (without quotes)
2. then plug in your external hard drive
3. in the command line type wbadmin get versions
4. note the version of the backup it tells you (something like 03/05/2008-01:31)
5. type wbadmin start recovery followed by appropriate parameters
For example to run a recovery of the backup from March 31, 2005, taken at 9:00 A.M., from drive C: and backup to external drive D, type:
wbadmin start recovery -version:03/31/2005-09:00 -itemType:Volume -items:c: -backupTarget:d: -recoverytarget:c:

more info and all the parameters are explained here
http://technet2.microsoft.com/WindowsServer2008/en/library/52381316-a0fa-459f-b6a6-01e31fb216121033.mspx

hopefully you'll restore it easily that way

Does this work with Vista, as opposed to WindowsServer 2008?

My problem is that the external drive in which the backup exists is now labelled j whereas it was labelled h when I made the backup!