I wouldn't install XenServer on VMWare, because Xen Server requires VT to run Windows guests OSs and VMWare doesn't not shows this extension to the guest OS (in this case XenServer), nor you can enable a "Virtual" VT from VMWare VM BIOS.
This will not stop you to install XenServer on a VM, but you'll have a not fully featured XenServer.
Probably you'll get a warning message during the installation, but well if this is what you want...
To install XenServer on VMWare Server you have to do what follow:
1) Update your copy of VMWare Server to 2.0.1 (or the latest one, if any superior to 2.0.1)
2) Create a VM for RED HAT Enterprise Linux 5 64bits (because XenServer is based on RED HAT Linux)
3) Let VMWare use your copy of XenServer ISO file as a cdrom
4) Run it
If you have any troubles during this process because your hardware does not supports 64bits then there is a problem with the processor reported by Windows XP to VMWare.
Update your Windows XP to the latest service pack and if this is not enough then reinstall VMWare Server after you updated you XP.
You don't need any particular set up or configuration to run XenServer on VMWare.