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Enterprise and Ultimate editions of Vista include a feature that provides data volume encryption called BitLocker Drive Encryption. Suggestions that BitLocker contains a backdoor allowing law enforcement agencies automatic access to encrypted volumes have been robustly denied by Microsoft.

But that doesn't necessarily mean the availability of Vista will mean the widespread adoption of disc encryption technologies that will frustrate law enforcement investigations in computer crime, including trafficking in images of child abuse, computer hacking, industrial espionage and other offences.

For one thing, in two of its three modes of operation BitLocker requires a cryptographic hardware chip called a Trusted Platform Module and a compatible BIOS. These chips are yet to become widely available much less deployed. The third mode requires a user to insert a USB device that contains a startup key in order to boot the protected OS.

That means law enforcement officers need to get into the habit of seizing USB keys as well as PCs in the course of conducting a raid. Brian Karney, director of product marketing at Guidance Software, said the computer forensics firm had worked with Microsoft on BitLocker and that it knew of "no backdoors".

Getting to machines while they are still turned on and taking a forensically sound copy is an option even in the absence of USB Keys, Karney explained. "Even though the logical volume is encrypted the OS works on top of an abstraction layer. We can see what the OS sees so that it's possible to acquire data on a running Vista machine even when it is running BitLocker."

In cases when a consumer machine running Vista happens to be turned off at the point its seized, a password is needed. However, in corporate environments a BitLocker recovery key can be used to allow examination of target devices.

In some ways, the issue boils down to who is more knowledgeable about the use of encryption or other security technologies: investigators or the targets of investigation, an issue far from restricted to use of encryption technology in Windows Vista.

"We're seeing the same concerns with Vista as we saw with XP over the idea that built-in encryption features might frustrate law enforcement efforts. In practice XP has not proved to be a problem for computer forensics and we don't think Vista will be either," said Bill Thompson, director of professional development and training at Guidance Software. "Sometimes people use file wiping utilities or other tools but often they are not configured properly. People accept the default settings, which can leave fragments of data."