Summary - You receive "There is an error in XML document (line, position). ' ', hexadecimal value 0x01, is an invalid character. Line, position position" , attempting to generator HTML reports.
A hotfix is available to solve this issue which is a requirement for support of MOM 2005 SP1 on SQL Server 2005 (including SP1). It's worth noting that this hotfix needs to be applied to the Reporting database server and not the Management Server.
For a list of all required KB article for MOM 2005 SP1 support on SQL Server 2005 (including SP1) see http://support.microsoft.com/?id=917615