Some PCs are more difficult to run Linux on than others -- it's all about the drivers. Some laptops are available with Linux pre-installed, but plenty aren't -- even though they may make great Linux PCs. The Ubuntu Certified Hardware database helps you find Linux-compatible PCs.
Most computers can run Linux, but some are much easier than others. Certain hardware manufacturers (whether it's Wi-Fi cards, video cards, or other buttons on your laptop) are more Linux-friendly than others, which means installing drivers and getting things to work will be less of a hassle.
Linux veterans probably know about this already, but I've had a few people ask me how to find Linux-friendly laptops lately, and my answer is always the Ubuntu Certified Hardware Database. Even if you aren't running Ubuntu, it will tell you which laptops and desktops from Dell, HP, Lenovo and others are most Linux-friendly.
If you're looking for a Linux-compatible PC, this is a great place to start. If it isn't in that database, though, be sure to Google around -- there may be forum threads dedicated to getting your laptop of choice set up with Linux.