What happens if the user tries to convert the file directly to PDF from Outlook (either via PDFMaker or Adobe PDF Printer) or from their web-mail client (printing to the Adobe PDF Printer)?
Trying to figure out how XPS & the XPS viewer are necessary in this workflow.
What error message are you getting when the Adobe PDF printer crashes?
Have you updated Acrobat to 9.5.1?
My guess is you are using AA9.0 which is not compatible with 64-bit systems. Do the update and then try again.
Some useful reference material.
Acrobat 10.1 adds PDFMaker support for these Office 2010 64-bit applications: Word, PowerPoint, Excel, and Outlook.
Acrobat 9 and earlier users can create Adobe PDF files using the Adobe PDF printer.
For more information, see PDFMaker unavailable in 64-bit Office 2010.
n.b., Acrobat 9 just went to 9.5.2