What version of PHD are you using Roger ?.
With the drivers being Astrosoft, there may be a easy fix. Email Tom with the same problems as see if theres a fix already in place. Check for latest drivers.
http://www.astrosoft.be/qhy5.asp
I have duplicated your condition on my vista laptop and XP desktop Roger.
Looks to be a driver or software bug.
I'll do some asking and digging around to see whats the problem.
OK, you dont have the latest driver from Astrosoft, so get it here
http://www.astrosoft.be/QGUIDER/CMOSDLL.DLL and replace the original CMOSDLL.dll file in the PHD directory.
Thats it, have fun guiding.
Theo