Dell will charge customers up to $50 to factory install Windows XP on some PCs after Wednesday, according to the company's Web site.

Buyers of the low-priced Vostro line of desktops and notebooks will pay $20 to $50 more for Windows XP Professional installed as a "downgrade" to Windows Vista Business or Vista Ultimate than they would for Vista only.

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To meet Microsoft's June 30 end-of-availability deadline , Dell will stop pre-installing most versions of the seven-year-old operating system after Wednesday. However, it will still be able to ship PCs with XP by taking advantage of the downgrade rights built into Vista Business and Vista Ultimate. Downgrading lets Dell install Windows XP Professional in lieu of Vista, although the newer OS is still shipped with the machine so that buyers can, if or when they want, transition from XP to Vista.

Vista Business and Vista Ultimate are the only generally-available editions that allow downgrades, and can be downgraded only to Windows XP Professional. By Microsoft 's licensing terms, the less-expensive XP Home cannot be installed as a downgrade.

In late April, Dell was the first major computer maker to announce it would downgrade Vista in order to continue installing XP after June 30. A few days later, Dell's global small- and mid-sized business software manager, Jenni Doane, used a company blog to say that Dell would offer free downgrades on Latitude, OptiPlex and Precision systems, but would charge what she termed a "small fee" on Vostro systems.

In fact, Dell has priced Windows XP downgrade options the same as Vista-only selections on its Latitude notebooks, OptiPlex desktops, and Precision desktops and notebooks.

Desktops and notebooks in the low-cost Vostro line, however, carry a downgrade surcharge, an examination of Dell's Web site revealed.

Adding Vista Business to a Vostro 1000 notebook, for example, costs an additional $99 above the price with the default operating system, Vista Home Basic. Selecting the downgrade option -- Windows XP preinstalled and Vista Business installation media in the box -- costs $149, however, a $50 downgrade surcharge.

Downgrading from Vista Ultimate comes with a smaller add-on charge, although the Vista OS selection itself is pricier. To add Vista Ultimate to a Vostro 400 desktop, for instance, costs $149 more than the default Vista Home Basic; choosing to downgrade from Ultimate -- Windows XP pre-installed and the Vista Ultimate installation disc in the box -- costs $169, a surcharge of $20.

On its consumer PC site, Dell doesn't specify whether it is slapping downgrade fees on three systems that can be configured with XP: the 630 and 720 H2C desktops and the M1730 notebook, all part of Dell's XPS high-end line. Dell has said that it will not offer downgrades for any "currently available Inspiron laptops and desktops." Inspiron is Dell's best-selling consumer brand name.

Dell did not respond to a call asking for more information about downgrade charges for its consumer PCs.

Although Microsoft will stop providing new XP licenses to computer makers after June 30, and stop selling boxed copies of the operating system to retailers on that day as well, it has relaxed its own availability rules twice in the last two months for some hardware categories.

In April, it extended XP's availability until June 2010 for light and inexpensive sub-notebooks such as the One Laptop Per Child XO and the Asustek Eee PC. Two weeks ago, it did the same thing for a new class of low-cost desktops dubbed "net-tops. "