Media Center Extender hangs on "Creating a user account..."

I'm trying to set up my XBox 360 as a media extender for my Windows XP
Media Center Edition system (running SP 3, and Media Center 2005
rollup 2). It goes through the process just fine until it gets to
"Creating a user account..." on the PC side. Then the hard drive
light starts flashing, and doesn't stop. I let it sit there for
several hours, and it continues to hang at that point. It -has-
actually created a user account (user name "MCX1", under the group
"MCRD Users"), so I have no clue what else is causing it to hang.
While this is happening, the XBox 360 is showing the "MediaCenter /
contacting..." screen.

When I press cancel on the PC, the hard drive continues to crunch
endlessly. I open task manager and check disk I/O, and the
"winlogon.exe" process seems to continually be performing hard drive
reads and writes. Using Process Explorer, I see that it's got a lot
of file handles open, but I'm not sure how to check which one it's
writing to. The only way I can get the hard drive light to quit
flashing is with a reboot; just logging off doesn't work.

I've also checked the event log, and nothing seems out of the
ordinary. One message around the time when this was happening was
"The WMI Performance Adapter service entered the stopped state." --
that's the only thing that I can see that seems a bit odd.

Connectivity all around is fine. Obviously the two systems can talk
to each other, because the MCPC verified the 8-digit security code
that the XBox 360 gave me. I have tried uninstalling and reinstalling
the Media Center Extender software on my PC. The exact same thing
happens, regardless.

Anyone have any clue what the issue might be, and more importantly,
how to fix it?

--Mike