Not too secure. There are tons of cracks and exploits for it already. Here's just one example of Vista activation:
Windows Vista Activation Cracked Yet Again:
Another day, another Vista activation crack
It was just a week ago that Microsoft's Jim Allchin was talking about Windows Vista security and how the operating system would fend off attacks from malicious code and hackers. Allchin made no mention, however, of the recent successful attempts at cracking Windows Vista's activation scheme.
Earlier this month, pirates found a way to spoof Microsoft's Key Management Service (KMS) server using a VMware image. The software hack allowed pirates to run copies of Windows Vista Business and Enterprise for up to 180 days.
The folks over at Engadget have come across another exploit that allows users to permanently activate Windows Vista using crack files and some registry trickery. The TimeStop Vista cracks only works on 32-bit versions of Windows Vista, so those looking to crack 64-bit versions of the operating system may be out of luck.
The crack effectively stops the countdown times to mandatory Vista activation and freezes the countdown timer at 43,200 minutes (30 days). The countdown timer will not reduce any lower than 30 days.
The makers of the crack note at the bottom of their "instruction manual" that "This article is for educational and informational purpose only." Microsoft likely isn't taking too kindly to this latest activation breach and likely already has a team working to patch up the exploit.
Despite Microsoft’s best efforts to shut down this latest exploit, it does leave us wondering just how secure this new operating system if it can be poked at and prodded this early after release.