Most laptops these days come preinstalled with one of Windows Vista's many incarnations. Unfortunately, a lot of laptop buyers have negative issues against Vista, and would like to use a different operating system.
One alternative is the Linux operating system. Since Linux is open source and therefore free, you can save a great amount of money on the operating system alone. Just search for the price difference between a Linux ASUS Eee PC and a Windows XP ASUS Eee PC. Needless to say, the difference amounts to at least a hundred dollars and that's a big enough savings you can otherwise use to buy other peripherals, accessories or software.
The only problem with Linux is that it is not as user friendly as Windows or a Mac OS. It is not very easy to install new software and aside from the Synaptic Package program, most installations have to be done using command prompts, which to most users will remind of the DOS era.
Linux actually has a number of versions or variations called distributions or distros. The most popular distributions are Ubuntu (and its other variations such as Xubuntu and Edubuntu), Xandros, SUSE Linux, Red Hat, and even Linpus which is being used in small laptops similar to the ASUS Eee PC.
As of now, there are still a great number of laptop owners that use Windows XP. Some people who bought laptops pre-installed with Windows Vista even downgrade to Windows XP. They claim that Windows XP uses less computing power as compared to the Vista and that it saves them from any compatibility issues.
So there you go. These are the commonly used operating systems on laptops. Each has its advantages and disadvantages. The important thing is you fully understand as to what purpose you will use your laptop for, and from that you can decide the right operating system for it.