Problems with AT&T Global Network Client

Hi all,
I don't know if that the right place for my problem, but I hope somebody can help me. I have a strange problem using the At&T client. I seems that it has something to do with the IKE Port. I get the following errors:
[0601105 19:42:54] IKE: Error binding to Listen socket (rc = 10013)
[0601105 19:42:54] IKE: Socket port number requested: 500
[0601105 19:42:54] IKE: Init Listen Thread Failed
[0601105 19:42:54] IKE: Error initializing the IKE routines 103
[0601105 19:42:54] IOCTL: Logon Failure, Error using sockets during IKE Init.

The client runs on a WinXP SP1 machine. Netstat didn't show, that port 500 is already used.

I have no idea what to do.

Thanks for your help,

Oli