It's clear that people are scooping up XP due to it's pending scarcity.  What's telling is that buyers have no reservations about purchasing the almost seven year old XP over the more recent Windows Vista.

Another view doesn't help
An unsorted view of the software list shows that the full version of XP Home checks in at No. 16 whereas the full version of XP Pro sits at No. 12.

Mac OS X is the top selling operating system
Apple's Mac OS X Leopard is the top selling operating system when sorted by operating system and No. 7 on the bestsellers software list.

Macs continue to be top sellers
In case you were wondering, four of the top five top selling desktops are Macs whereas two of the top five selling notebooks are Macs (note that of the top five notebooks, two are running Linux!). I suppose it's a sign of the times as people are no longer locked into a PC running Windows.

Note that these rankings may change as the list is updated hourly.