SEVERAL NEW COMPUTERS unveiled in recent months have something in common besides their budget-minded price tag: All come preloaded with the Linux operating system instead of Microsoft's Windows. Until recently, Linux catered to techie do-it-yourselfers, who loaded the free system themselves and were free to distribute or modify it as they wished. But pre-installed versions of Linux are now turning up on PCs and laptops aimed at more-casual users and available at mainstream outlets such as Wal-Mart and Sears.com. These "just enough" computers are worth exploring if you need e-mail, Web browsing, word ...