I have the same problem and is now looking for an answer too. Mine installation is windows 2003 SP2 and it is working after the installation while it goes problem after a certain period of time. During that period, I have only applied recommended patch from microsoft. Guess that the problem should be caused by one of the patch coz one of our XML parsing application also not working after the patch installed.

Anyone have clue ?

I am currently using MSXML 6.0 Parser (KB933579) and .NET framework 2.0 SP1 whereby my BackupExec is 11 Rev 6235. The update I have installed is SP 1 and Hot Fix 27.

I would imagine that someone from Symantec would be monitoring this message board and helping users with problems. I am not shocked to find that this problem was posted here over a month ago and remains unanswered.

I am also having this same issue. I had trouble with a ver 9 and paid for support. Of course the issue was NEVER solved and although I spent hours upon hours on the phone with a tech from symantec (and paid for the support on top of that!), I was not able to get a discount on the latest software. I then purchased 11d OUT OF POCKET and put that on my server. It ran for two months and now I have this same error. "An instance of an XML Document is not valid. Check that Microsoft XML Core Services (MSXML) 4.0 are installed on the computer."

So why is it that we tend to stick with products from a company that can't help us support them?

Try installing Microsoft XML Core Services (MSXML) 4.0 sp2. You can download it from the following link: http://www.microsoft.com/downloads/details.aspx?familyid=3144b72b-b4f2-46da-b4b6-c5d7485f2b42&displaylang=en