Experts say Vista may be just as accessible to hackers as the Windows operating system it replaces.

Gerhard Eschelbeck from security company Webroot Software told The Guardian:

"We want to make sure that users understand the system's limitations and caution them that Microsoft's anti-virus programs may not fully protect them."

It says the new Windows Defender system failed to block 84% of viruses in tests.

And it's also being attacked by the environmental lobby because many users will have to buy new PCs to run Vista.

Sian Berry of the Green Party said:

"There will be thousands of tonnes of dumped monitors, video cards and whole computers. Future archaeologists will be able to identify a Vista upgrade layer when they go through our landfill sites."

A version of Vista for business users has been available since late last year.