Safe Mode is an alternative way to start your computer. Normally you start your computer in Normal Mode which means as Windows starts all the applications and services you have indicated you want to automatically start when Windows starts all start up. Most people do not realize as they install different programs they may be having parts of them start automatically at Windows startup. You can go to msconfig (typed in the search box in the Start menu) and see what all you have starting when you start Windows. If you know you don’t want stuff auto started you can uncheck them.
When viruses and spyware get on your machine they will add themselves to auto start to make them hard to remove. Also sometimes settings in Windows get messed up and your Windows will not start.
Safe Mode is an alternative to starting Windows and it just starts the basic Windows program and services and no extras. This allows you to get in Windows and make repairs or to disable programs or services that no longer work. You normally start plain Safe Mode which just lets you do basic Windows items and no networking, etc, but there is a Safe Mode with networking.
To get in Safe Mode if you are running Windows XP or Vista (or earlier Windows) you will start pressing the F5 key repetitively as soon as you hit the power switch. It will lead you to a menu with an option for Normal Mode, Safe Mode and other modes. You will choose Safe Mode. If when you try this you get to a normal Windows screen turn off the computer and try again pressing F5 lots of times rapidly as the machine starts. It may take several tries.
If you are using Windows 7 there is an extra step. You will turn on the machine immediately hitting F5 lots of times rapidly. Then it will stop at a screen asking if you want to start Windows normally or using diagnostics. There is a third option you will do which is hitting F8 for advanced options. Pressing F8 will get the screen for Safe Mode, Normal Mode and other options..