In a recent podcast Steve Gibson of grc.com has drawn attention to a detailed report by engineers at Symantec who demonstrate that Windows Vista contains a completely virgin network stack that has been programmed from the ground up.
The Symantec software engineers have monitored the behavior of the new stack through a series of beta releases and have documented that it contains most of the basic bugs and security holes that have long since been fixed in other stacks - even the Windows 95 stack. Since it has not had a chance to mature and develop in the wild, the likelihood that it contains new, uncharted holes and errors is very high. Some have already been found.
Gibson stresses that the ramifications for the security of the new stack are disastrous. Towards the end of the podcast Gibson suggests that if you must use Vista you should just install it as second OS and "only play with it on weekends."