Hewlett-Packard said in June it would offer free upgrades to Windows 7 for customers who buy qualifying PCs pre-installed with certain editions of Vista in the U.S. and Canada. Customers need to go to HP's to check upgrade eligibility. Qualifying customers will receive Windows 7 upgrade disks starting Oct. 22, when the new OS becomes generally available.

Companies including Lenovo and Dell are also offering free upgrades.

Mainstream laptops under $300 are rare, and for such a low price, the Presario laptop is packed with features. A free upgrade to Windows 7 might have given additional impetus for users to buy it.

The laptop should give better graphics and application performance than netbooks, which are generally less powerful systems with smaller screens and keyboards.

Wal-Mart is expecting a rush of customers and it could run out of stock quickly. "We expect this one will be quite popular," a Wal-Mart employee wrote in its blog entry announcing the offer.