Rootkits, Bootkits, Viruses & Windows VISTA

Rootkits are the worst nightmares that a computer user ever faces (even security administartors). although,they have been quite popular on win2000,win XP,Win 2003.But what about rootkits etc. on vista.Is vista secure from rootkits,unsigned drivers & viruses etc.?

What has been added or modified in vista to secure it from above attacks?
Microsoft has been quoting that only signed code will be allowed in vista, thus making vista more reliable and secure.Also, it is being touted again & again that vista has been redesigned again from scratch keeping security in minds and it is Microsoft's "most secure ever" operating system.
Does vista really stands the claims?

What about security of booting of an OS? Unsecure Booting can lead a system from trusted to untrusted stage.
It's likely that unwanted malicious code can attach during the booting process.What about if kernel is patched during booting or offline ? IS vista secure from this?

Microsoft thought about this and developed the Bitocker System but once again hell breaks loose, it's only available for high-end Vista Versions ( Ultimate and Enterprise), so probably the rest of us are left alone to face the attacks. What was Microsoft thinking when it cleared the idea that low price version would have security features disabled?

we have successfully used the boot process trick to circumvent system security.(Vbootkit)

So finally, whats the difference between win XP & vista, rather than xp was released several years back !!!!

Is Vista really secure ????