This is the same issue I am having. see KB933666. I installed Office 2003
service Pack 3 on all my office machines and the new service pack features
security fixes that block all *.wk4 files. This is the same patch that has
been applied to Office 2007. This is the format, I think your are exporting
out of the AS400. If it is, I would uninstall office 2007, make sure the
folder is deleted then reinstall Office 2007 after a reboot. Then beware of
the security patches you install. Let me know if this was helpful.