In his second comment here, PaulH suggests changing registry settings, providing this link.
I'm not sure editing the registry is really something I want to do, because copying files to the handheld was working fine until late yesterday, and I doubt any change was made to the registry to cause that problem to commence, so I'm not at all sure that changing the registry will fix the problem.
At any rate, I wanted to compare the values I have in the registry with what is shown on that msdn page linked above. The "USB Function RNDIS Client Driver Registry Settings" section of the page references several values stored in the Registry below HKEY_LOCAL_MACHINE\Drivers
However, when I Start > Run > "regedit" on my Windows CE handheld device, it doesn't know what RegEdit is.
I even looked on my PC for such a branch, but I have no "Drivers" folder beneath HKEY_LOCAL_MACHINE; all I've got are:
BCD000000000
HARDWARE
SAM
SECURITY
SOFTWARE
SYSTEM
So how can I examine my registry settings on the handheld device? I thought maybe my software was too old (pre-dated the registry) but the page referenced above is for Windows CE 5.0, so that's obviously not the case.
I downloaded the "remote registry editor" which PaulH recommends below. However, here he recommended changing the registry setting from RNDIS to Serial_Class yet it is already set to Serial_Class:
Does this mean I should try to change it to RNDIS and see what happens, or am I doomed?
Windows CE doesn't come with a registry editor (as you discovered). You can download one, use the Registry API, or (if you can get activesync working) use a remote registry editor.
The registry layout on the WinCE device only vaguely resembles the on on your PC. Don't look for any real consistency there.