Amazing. I bought that key in 2004 and never had a problem.

Your suggestion made it work! Thanks.

I am running XP.
I have FSUIPC 3.98a
I run Microsoft Security Essentials
XP firewall is on.

Here is the FSUIPC log:

Verifying Certificate for "C:\Program Files\Microsoft Games\Flight Simulator 9\MODULES\FSUIPC.DLL" now ...
   SUCCESS! Signature verifies okay!
Running inside FS2004(original release)
User Name="George"
User Addr="xxxx"  ( I removed my address for this post )
FSUIPC Key is provided
WideFS Key is provided
Module base=61000000
ClassOptions: UIPCMAIN=FF7F, FS98MAIN=FF7F, FS2KMAIN=FF5E
WeatherOptions(Orig)=40003605[40003605]
InitDelay: 0 seconds
WeatherReadInterval=4
LogOptions=00000001
DebugStatus=255
    18359 System time = 21/07/2010 22:15:09
    18375 C:\Program Files\Microsoft Games\Flight Simulator 9\
    18375 System time = 21/07/2010 22:15:09, FS2004 time = 12:00:00 (00:00Z)
    29343 C:\Documents and Settings\George\My Documents\Flight Simulator Files\DC-3 @ KLZU.flt
    29515 AIRCRAFT\MAAM-SIM DC-3 Passenger\MaamSim_DC3.air
    29515 Aircraft="Douglas DC-3A DCA bare metal MAAM"
    42515 C:\Documents and Settings\George\My Documents\Flight Simulator Files\UI generated flight.flt
    43312 Clear All Weather requested: external weather discarded
    49796 Advanced Weather Interface Enabled
    82718 WeatherOptions set, now 40003605 (timer=0)
   229828 System time = 21/07/2010 22:18:40, FS2004 time = 22:17:44 (02:17Z)
   229828 *** FSUIPC log file being closed
Memory managed: 0 Allocs, 630 Freed
********* FSUIPC Log file closed *** *******

I have the latest version of FSUIPC (I just reinstalled it and reregistered it). No WideFS as this is all on one computer.
I have also installed .NET 3.5 SP1 and all updates.

8:30:01 PM Starting Plan-G, build 0.9.1.423
8:30:01 PM FS9 is installed
8:30:01 PM FS9 database is built
8:30:01 PM FSX is installed
8:30:01 PM Selected data set is FS9
8:30:08 PM Starting map
8:30:09 PM Initialisation complete.
8:30:29 PM RibbonButton_Click_ConnectFS
8:30:29 PM FSUIPC.Connect
8:30:29 PM Error opening FSUIPC: FSUIPC.FSUIPCException: FSUIPC Error #7: FSUIPC_ERR_VERSION. Incorrect version of FSUIPC.
   at FSUIPC.FSUIPCConnection.Open(FlightSim RequiredFlightSimVersion)
   at FSUIPC.FSUIPCConnection.Open()
   at FS.FSUIPC.Connect()