I was thinking about this also at the weekend (I really should get out more) about how to reliably backup my Exchange 2003 SP1 Virtual Machine running at home. Now it only supports five users and a few resource mailboxes, but now that my wife's email is on it, it suddenly becomes mission critical of course. Since installing it a couple of weeks ago, I'd been relying on a scheduled backup of the VHD files from the host itself, but this isn't sustainable on a long-term basis due to disk space requirements.
One of the first hurdles to overcome was hitting Event ID 8017 when using NT Backup to do a full online backup of the first (and only) storage group. The Event Log says "The operation failed. Consult the backup Report for more details. This didn't give a lot of further information either.
However, after some searching through the internet, I found the answer. It's simply a case that I had circular logging still enabled for that storage group (even though I swear blind I had turned it off). Under Exchange System Manager, select the First Storage Group, right click and de-select "Enable Circular Logging". After this, it will be necessary to cycle the Exchange Information Store service - that was the "trick" to making it work.
Hope that helps someone.