Access to the resource [mapped drive letter] has been disallowed.

We've recently upgraded our network - Win2000 Servers to Win2003, desktops to
WinXP. Now some groups are restricted from accessing drives when they type
the drive letter in the address bar. The error message reads:

Address Bar
Access to the resource [drive letter] has been disallowed.

The group does have other GPOs that are limiting access to such things as
Run and the Control Panel. The error appears whether they try to access a
local drive (c or a mapped drive (such as q:\). It's particularly
troubling as I can't narrow down how to begin troubleshooting this.

I've looked through GPOs and can't find anything that would limit drive
letter access in the address bar. Any help?