I can't tell you for sure what's causing the reboot. However you may try to:
- reset the CPUID mask to default in the VM's settings (Options -> CPUID Mask -> Advanced)
- add scsi0.virtualDev = "lsilogic" to the VM's configuration file. The LSI Logic Parallel adapter is the default controller type for Windows 2003 (rather than BusLogic) After editing the .vmx file you need to either remove and re-add the VM from/to the inventory or reload it from the command line (see http://kb.vmware.com/kb/1026043) To ensure the proper drivers are present, temporarily add an additional virtual disk to the VM and select the "LSI Logic Parallel" adapter for it. Make sure you add the additional virtual disk and let Windows detect it before editing the .vmx file!
André