Even though it has had its own problems of late, Windows XP remains the most-used version of Windows. The newest data from Web metrics vendor Net Applications Inc., for example, pegs XP as driving 73% of the personal computers that went online last month, five times the nearest competitor, Microsoft Corp.'s own Windows Vista.

Which is why an impending deadline five weeks from today is important.

According to Microsoft, June 30 is the last day it will permit retailers and OEMs to sell the nearly-seven-year-old operating system.

You'll have questions as that date approaches, including whether the deadline will drive up prices (gouging, anyone?). And we plan to have the answers, starting with this FAQ and continuing through the end of next month.

How long until Microsoft shuts off the XP spigot? Five weeks from today is the last day Microsoft will officially allow retailers to sell the old operating system and let major computer makers -- called "OEMs," for "original equipment manufacturers" -- to sell PCs with XP pre-installed.

Monday, June 30, is the EOL -- or End-Of-Life, a term Dell Inc., not Microsoft, has publicly used -- for XP's retail and OEM availability.

So what's the June 18 date I've heard about? That's the day that Dell has said is the last possible day for its customers to buy a machine running Windows XP. "To meet Microsoft's June 30 last-day-to-ship OEM Windows XP deadline, June 18 is the last time to purchase a Dell laptop, desktop, or workstation with an OEM Windows XP license," Dell says on its Web site.

Other big-name OEMs, such as Hewlett-Packard Co. and Lenovo Group Ltd., have not announced cutoff dates, but they must also abide by Microsoft's rules that no XP-based system can ship after June 30.

Are there any loopholes? Sure, and they're big enough to steer the Titanic through. A "downgrade" clause in Microsoft's guidelines for OEMs lets computer makers install Windows XP Professional -- but not the more common and less expensive Windows XP Home -- on new PCs at a customer's request when those machines are ordered with Windows Vista Business or Windows Vista Ultimate.

Dell took advantage of the clause to announce last month that it would use the downgrade rights of Vista Business and Vista Ultimate to install XP Professional free of charge at the factory. Assuming customers want to, they can later use the included Vista installation media to upgrade from XP Professional.

Other vendors are doing similar things. HP, for example, also offers a free at-the-factory XP Professional downgrade option on some systems sold with Vista Business.

Can I still buy Windows XP? Absolutely. In fact, we've started tracking retail availability and pricing at three popular online technology outlets -- Amazon.com, Buy.com and Newegg.com -- to gauge whether the operating system is in stock and what it's selling for.

For the three-day stretch leading up to Monday, May 26, all three outlets have Windows XP Home OEM -- the least-expensive version of the operating system, but also the one that comes with the most restrictions -- in stock and ready to ship for those who want to downgrade a Vista machine on their own, or who prefer to have a backup license in hand.

The prices for May 26, shipping included, were $95.15 at Amazon.com, $100.24 at Buy.com, and $89.99 at Newegg.com.

We'll revisit those stores on a regular basis and report our findings in future FAQs or stories in the run-up to the June 30 EOL deadline.

Will I be able to buy Windows XP after June 30? Affirmative. There's already a healthy market for the workhorse on eBay, the online auction site that sells both old and new stuff. That won't disappear overnight.

Again, we've started tracking eBay's listings for a couple of Windows XP metrics: How many items pop up when a search using the string "windows xp" is run on the site, and the current lowest "Buy It Now" price for Windows XP Home OEM.

We tried several different searches on eBay. During the three days prior to May 26, we found 1,804 results for "windows xp;" 1,080 for "windows xp" narrowed down to listing strictly in the "Software" category; and 675 for "windows xp" narrowed to not just the Software category but by "operating system" and "windows." The auction site's lowest prices, shipping included, for a legitimate copy of Windows XP Home OEM for the three days were $91.75 for May 23 and 24 (that is, Friday and Saturday), with a slight uptick to $92 on Sunday.

We'll monitor eBay's XP listings and prices to see if, for example, more sellers start flogging the operating system as June 30 approaches, and whether prices move as the end draws near.