I have a few ideas about what could be related.
This could be a windows XP specific issue.
It could be because of crossfire.
I only get hardware errors on my slave GPUs.
btw, do you have an option to turn off hardware checking in the newest version?
Its not OS related, Linux users are getting it. It is also not Crossfire related, as people who turn off Crossfire and people with single GPUs are getting it.
Turning off HW checking would do nothing, as a high number of errors are also causing potential pool shares to evaporate (they're getting about 3/4th as many).