The OS is what makes then PC, er, operate. Windows is the most popular OS ever and we're now seeing a transition, albeit it slowly, from Windows XP to Vista. Microsoft says it's "retiring" XP in 2008 although it will continue to support the product for some time to come.

But there are alternatives. Linux is the most popular alternative OS for the PC - it's also free! - and Apple Macs have their own Mac OS, currently Version X.

Software applications such as word processors, spreadsheets and games, have to be written for a specific operating system so, alas, you cannot run a Windows game under Linux. However, there are many, many free Linux applications should you want to explore that OS, although there are a great deal more free programs for Windows, too.