In order to troubleshoot your computer or know what programs you'll be able to install, you must be able to tell which operating system you have. An operating system, or OS, is software that tells your computer hardware, such as the central processing unit (CPU) and hard drive, how to interact with other programs. Each operating system has a different user interface which allows you to interact with the computer -- things like menu options and program icons -- but the main difference is in how the different operating systems control your computer.
Look for a "Start" button, which is usually in the lower left-hand corner of your screen. If you see this, you are running a version of Windows. Click "Start," then "Run." Type "winver" (without quotation marks) and click "OK." A window will pop up with information about your operating system.
Look for an apple icon in the upper left-hand corner of the screen. If you see this, you are running a version of Mac OS. Click the apple, then click "About this computer" or "About this Mac" to see what version of Mac OS you're running.
Restart your computer, if you can't find the operating system using the above steps. When the computer boots up, the screen will show what operating system you're using.