Computer users were warned off downloading a test version of the latest Windows release today, as millions more copies could be accessed in the coming days.
IT giant Microsoft released a "beta" version of Windows 7, which allows expert users to try the product and highlight any glitches.
A surge in demand led the firm to suspend a 2.5 million limit on the number of copies available until January 24.
But today the firm stressed the product is only aimed at computer experts. Microsoft offers no technical support to those testing the product.
Laurence Painell, a product manager for Microsoft, said the company had been "genuinely surprised" at the number of users wanting to test Windows 7 Beta.
But he added: "We don't recommend that if you've got one PC at home and you're an average consumer that you install a beta product because it's unfinished. It can have bugs and issues with it."
John Bogue from consumer magazine Which? Computing said the average PC user "should not go near it".
"They make it quite clear there's no point if you're not completely self-sufficient with computers," he said.
"You shouldn't go near it otherwise, because anything beta by definition is not a complete product. It's a testing product and it could crash any machine at any time.
"There's no guarantees with a beta, it could completely ruin your computer.
"It's only for the enthusiasts who have a spare computer to install it on. You wouldn't ever install it on your main machine."
The release is a routine way for Microsoft to test a new product, so users can report any glitches before mainstream release.
A final version of Windows 7 should go on sale before January 2010.
The company says it is designed to make everyday tasks easier and to allow better access to entertainment content such as music and films.