Some used PCs are being purchased not for the hardware but because people want to get their hands on a copy of Windows XP, according to industry analysts and hardware suppliers.

A lot of PCs have been disposed of because Microsoft's Windows Vista OS has created a demand for PCs with greater hardware capabilities, according to a study released by Gartner on Tuesday. In 2007, about 197 million PCs were discarded, of which 44 percent were put up for sale secondhand.

Many consumers and businesses prefer used PCs with Windows XP because they still have older copies of software programs that often don't run properly on new PCs running Vista, said David Daoud, research manager at IDC. For example, consumers may need to buy used PCs with Windows XP to run older accounting software that is incompatible with Vista.

Similarly, many businesses have been slow at adopting Windows Vista because of uncertainty surrounding whether it would work well with existing software. Notably, Intel said earlier this year that XP will be the dominant OS for most of its employees for the foreseeable future.

Rescuecom, a computer repair firm, has resold XP-based PCs to companies that use software that is incompatible with Vista and which would require a substantial upgrade to become compatible.

Harry's Shoes in New York City specifically requested XP-based machines from Rescuecom when purchasing PCs, as it uses multiple programs that would need upgrades to be compatible with Vista.

"The other main reason we see people staying with XP is for standardization. Having five PCs that are Vista and five XP can create training issues and compatibility issues," said Josh Kaplan, president of Rescuecom.

Buying a used PC is also an inexpensive way for price-sensitive users to secure an XP license, Daoud said. The other alternative would be to buy a new business-class PC with Vista and then downgrade it to XP, which can be expensive.

While lower prices make used PCs attractive, users need to ensure they are buying hardware with a genuine XP license. Many resellers are bundling illegitimate copies of XP so they can sell used PCs at cheaper prices, said Charles Smulders, managing vice president at Gartner and a co-author of Gartner's study. Loading illegitimate software keeps the prices of used PCs low, as it reduces the software overhead for resellers.

It is smarter to buy used PCs made by large PC makers, as they come loaded with legitimate copies of software like Windows, Kaplan said. Microsoft usually provides certificate of authenticity (COA) stickers on top of branded PC hardware that helps users identify a genuine copy of Windows XP.