Before rebooting in safe mode, disconnect all drives from CDs, DVDs and floppy disks. If you use flash cards or external hard drives, startup items, remove them too.
After that select restart in the start menu. There, click the "shutdown" in the next menu choose "Restart".
If you have only one Windows operating system, after reboot press F8 and keep it pressed until the screen appears the Windows logo. You can not hold down, and press F8 constantly, until, until you see the logo. If it is already there, and you have not had time to press F8, wait for loading the operating system and reboot again, this time pressing F8 in time.
If your computer has multiple operating systems, then after you see the boot loader and menu system that should be loaded now select the Windows system you want to start in safe mode, and then press F8 just as in the previous step.
After the menu appears on system boot Windows in safe mode. There will be several options. Using the keyboard, select the one you need. If you don't know what mode you need, then just choose "Safe mode" without any additional parameters.
If you need to perform some changes, you have to log in using an account that has administrator rights.