Windows 7 is vulnerable to 80 per cent of viruses, a security software vendor has warned.

Sophos’s senior security advisor, Chester Wisniewski, reported in his official blog that during a trial of Microsoft’s new operating system on a clean PC, the machine became infected with eight out of the ten viruses tested.

Engineers loaded a full version of Windows 7 without installing an anti-virus package, and configured it to follow system defaults for UAC (User Account Control).

According to Wisniewski’s blog, ten unique samples which arrived in the SophosLabs feed were selected “to see how well the newer, more secure version of Windows and UAC held up.”

“Unfortunately, despite Microsoft's claims, Windows 7 disappointed just like earlier versions of Windows. The good news is that, of the freshest 10 samples that arrived, 2 would not operate correctly under Windows 7,” he wrote. “Lesson learned? You still need to run anti-virus on Windows 7.”