i am having problems with my OpenMG software (for Sony's Net MD
player). Although I have had the software installed correctly before,
it has recently started playing up. I uninstalled the software and the
driver, and have tried reinstalling, but everytime I try to install
the driver, it is not able to and an error message comes up saying
"netmdusb.inf Access is denied". Can someone please tell me how I can
install the driver and overcome this problem?

Uninstall all the OpenMG software and the drivers for the Net MD. 
Then, find netmsusb.inf in c:\windows\inf and delete it.  .. Then,
install the latest version of the OpenMG software and the Net MD
drivers.  Get that here:
http://www.musiclub.sonystyle.com/support.jsp
Select your model under software updates.

...  You know how to handle it from there, right? =)

Hope this helps.