JBoss AS 5.x is pickier about the ordering of entries in XML file. Either correct the ordering, or run with the option ??? (I know there is an option, but can't find it - perhaps someone knows it offhand).
The property to set, depends on what is wrong in the xml contents. If the order of the elements is incorrect, then setting -Dxb.builder.useUnorderedSequence=true in the run.bat/run.conf scripts. However, if the xml contains invalid elements which is causing the validation failure, then the workaround is to disable validation (and given that the forum search has been returning 0 results for whatever search term i enter, we won't be able to point you to a similar thread where this was discussed).
But as Peter mentioned, the real fix to this issue is to fix your xml to follow the schema.