If your VM is only reporting 100Mbps network speed, that is because you are using the VLANCE VMware NIC instead of the VMXNET NIC. Don't worry, it's a false indicator, your virtual NIC is still transmitting packets as fast as possible based on the PNIC or system bus speed, but you should upgrade to the VMXNET NIC wherever and whenever possible as it is a more efficient and slightly faster NIC than the slower-more-compatible VLANCE which is a virtualized old AMD PCNET Ethernet standby.
Regards,
Milton 