As your computer is booting, press the F8 key repeatedly just before the splash screen. You should get a bunch of different boot options from there such as:

Safe Mode: Loads minimum amount of processes needed to run the OS

Safe Mode with Networking: Loads a minimum amount of processes for the OS as well as the Networking functionality.

Safe Mode with Command Prompt: For more advanced troubleshooting it loads the minimum amount to run the OS as well as access to the command prompt.

Just let the PC boot normally when you want it to go into "normal mode", otherwise there is an option in the menu to boot normally.