How much is it going to cost to downgrade to XP on a new PC?

That depends on the computer maker.

Dell, for example, isn't charging anything for factory-installing XP on Latitude, OptiPlex and Precision systems, but is adding up to US$50 for the downgrade on the Vostro brand, its entry-level small business line, beyond what it would cost for the Vista license alone.

The downgrade fee for the three XPS machines, meanwhile, is US$20.

HP, on the other hand, doesn't add a downgrade surcharge for preinstalling XP in lieu of Vista.

What about the "XP tax" I've heard about?

The difference between XP in its usual form and XP as a downgrade is what some have dubbed the "XP tax," a surcharge, so to speak, derived from Microsoft's licensing requirements. After June 30, those requirements limit XP as a factory install to downgrades, and further limits those downgrades only when purchasing Vista Business or Vista Ultimate.

Here's an example of the tax in action.

At Dell, the Latitude D630 laptop comes standard with Vista Home Basic, the lowest-priced version of the OS in Microsoft's arsenal. But to qualify for a downgrade to XP Professional -- which is, remember, not just the only version that can be factory-installed but also the pricier of the two, Home and Pro -- customers have to opt for, at the least, Vista Business.

Vista Business is a US$99 add to the D630, according to Dell's online store customization tool, as is what Dell dubs the "Bonus" option, which in effect sells you a license to Vista Business but installs XP Professional.

In this case, then, the XP tax is US$99.

But it can be higher. Much higher. Take Dell's XPS 1730 notebook, a zaftig gamer's portable (10.6 pounds, minimum). By default, the lowest-priced XPS 1730 configuration is sold with Vista Home Premium, which doesn't allow for downgrading to XP. Instead, buyers with XP on the brain have to shell out an additional US$170 (which includes a $20 downgrade surcharge) for Vista Ultimate just to get XP.

The tax, of course, is a by-product of the price differences between versions of Vista. But while Vista Home Basic omits numerous features found in Vista Business -- see this Microsoft chart for the maker's take -- and even Home Premium lacks a few features included with Business, it's all moot if all you want is XP.

No wonder that some pundits, bloggers and analysts have said that XP's days are numbered.