Make sure you don't have any older versions of VNC running. Uninstall and clear the service entries in the registry if needed.
Go to the downloadpage http://www.uvnc.com/download/ and get the version called:
"Win32 UltraVNC 1.0.5 Setup" (yes, win32)
install it the normal way. Don't start VNC server just yet, if it did start already, kill the "winvnc" process via Task Manager (Ctrl+Alt+Del).
Then get the latest build, the zip called server_viewer_bin64.zip from http://sc.uvnc.com/105/specialbuild/latest_build/okt5/
also, while you're there, get the latest drivers.zip from that folder.
In the server_viewer_bin64.zip is a winvnc.exe, overwrite the one in your previously installed UltraVNC folder with this x64 one!
Also, unzip the drivers files from driver\xp64\ and run the install.bat in that folder after unzipping. You can also right-click on driver\xp64\driver\mv2.inf and choose "Install".
You need to run winvnc manually to enter the Admin properties, but you could also try and edit the ultravnc.ini manually. Mine looks like this (example):
[Permissions]
[admin]
FileTransferEnabled=1
FTUserImpersonation=1
BlankMonitorEnabled=1
CaptureAlphaBlending=0
BlackAlphaBlending=0
DefaultScale=1
UseDSMPlugin=0
DSMPlugin=
SocketConnect=1
HTTPConnect=0
XDMCPConnect=0
AutoPortSelect=0
PortNumber=6499
HTTPPortNumber=6498
InputsEnabled=1
LocalInputsDisabled=0
IdleTimeout=0
EnableJapInput=0
QuerySetting=2
QueryTimeout=10
QueryAccept=0
LockSetting=0
RemoveWallpaper=1
RemoveAero=1
DebugMode=0
Avilog=0
path=C:\Base\UltraVNC
DebugLevel=0
AllowLoopback=0
LoopbackOnly=0
AllowShutdown=0
AllowProperties=1
AllowEditClients=1
FileTransferTimeout=30
DisableTrayIcon=1
MSLogonRequired=0
NewMSLogon=0
ConnectPriority=0
AuthHosts=-:+194.109:+192.168.1
[ultravnc]
passwd=xx