Businesses are buying second-hand PCs running Windows XP as a cheap way to keep mission-critical applications running.
Companies are also buying old XP machines as they do not want to mix perfectly good existing XP machines with more expensive Vista machines when they need extra PCs.
Such mixed environments can cause operational and training issues.
According to Gartner, there were around 197 million PCs discarded in 2007, 44% of which are being sold in the market as second-hand items. Many of the machines were originally discarded by firms going for a complete Vista upgrade.
Some are now being sold on auction sites such as eBay and others through traditional reseller channels.
Consumers are also buying second-hand XP machines as some of their applications do not run on Vista either.
Charles Smulders, a Gartner analyst, has warned firms to check whether the second-hand PCs they are considering are running a legitimate copy of XP before buying them.
Microsoft has already extended the availability of Windows XP when sold with new PCs.