Walk into just about any computer store or log onto most Web sites that sell computers, and you’re sure to find a good number of laptops and desktops at rock-bottom prices. But those deals may not be as good as they seem on the surface, our lab tests show.

Take a close look at these “bargains” before you buy, because some computer manufacturers are replacing dual-core processors with single-core Pentium and Celeron processors to reduce costs. Others are using less-powerful Atom processors in systems that otherwise look like full-blown laptops and desktops. The result, our tests show, is more sluggish performance when you try to surf media-rich Web sites, watch videos, or play 3D games.

The bottom line when shopping: If you’re looking for a primary PC for general-purpose computing, avoid computers with single-core processors, such as some Atoms and Celerons. Also make sure you get more than 1GB of memory. Of course, if all you need is a low-cost netbook for general Web surfing and e-mail, with perhaps some word processing thrown into the mix, then an Atom-based netbook with a gigabyte or two of memory and at least a 160GB hard drive should serve you well.

Take a look at our laptop and desktop Ratings (available to subscribers only) to see how Atom and other low-end processors affect systems like all-in-ones and other full-blown computers. —Donna Tapellini