Dell on Wednesday said customers who want Windows XP preinstalled on a desktop or notebook will have to buy a copy of Vista and pay an additional $20 to $50.

Dell is offering XP computers under Microsoft's Vista-to-XP downgrade licensing program that allows customers who purchase the new operating system to legally install its predecessor on their PCs. Microsoft does not require computer makers to charge extra for installing XP, a decision Dell made to cover its own costs, spokeswoman Anne Camden said.

Dell is scheduled to officially start charging customers for XP on Thursday. However, the company plans to waive the fee for a "limited time" as part of a promotion, Camden said. She declined to say how long the promotion would last.

Once the fees go into effect, the charge will be added to the purchase of Vostro business notebooks and desktops and select models of the XPS gaming systems. The latter includes the XPS 630 and 720 desktops and the M1730 notebook.

Vostros will cost $20 to $50 more with XP, depending on the model and whether it's a notebook or desktop, Camden said. The XPS gaming systems will cost $20 more.

XP will also be available for Latitude notebooks, Optiplex desktops, and Precision workstations. However, Dell does not plan to charge buyers of those models for XP. The three product lines are typically bought by businesses, and the Latitude and Optiplex machines are often bought in volume.

Under Dell's downgrade program, buyers will have to purchase either the Windows Vista Business or Ultimate edition and would get XP Pro pre-installed. No other versions of XP will be offered. Customers will also get both operating systems on separate DVDs.

Microsoft's blessing of the downgrade program is an indication that the software maker may be counting on it to spur sales of Vista, which many businesses have rejected due to its hefty hardware requirements and incompatibilities with older applications. XP's official expiration was set for June 30.

The situation has raised questions about sales figures that Microsoft has provided for Vista. In April, the company said it had sold more than 140 million Vista licenses since the release of the OS early last year. Microsoft, however, has not said how many of those licenses have been downgraded to XP.

A number of government and commercial organizations have said they may bypass Vista altogether and migrate from XP to Windows 7 when the later becomes available in late 2009 or early 2010.