A vulnerability has been discovered in vmnat.exe on Windows hosts and vmnet-natd on Linux systems. The vulnerability in this component affects VMware Workstation 5.5, VMware GSX Server 3.2, VMware ACE 1.0.1, VMware Player 1.0, and previous releases of these products. The vulnerability makes it possible for a malicious guest using a NAT networking configuration to execute unwanted code on the host machine.
VMware believes that the vulnerability is very serious and recommends that affected users update their products to the new releases available at www.vmware.com/download or change the configuration of the virtual machine so it does not use NAT networking.
from [Spyware Warrior]
Iv'e been using VMWare for years. I implement the installers for most of our companies products and there is no way I could test the installers with running various combinations of Windows inside VMWare sessions. I could use Virtual PC as it comes with my MSDN subscription, but I think VMware is still a better product.