Other than attempting a reboot I cannot help a lot as rarely use Windows but ctrl+alt+delete used to work for log out and may give you a shut down option to try booting again.
If it's a brand new PC with Windows pre-installed then it's the initial setup that's failed. This isn't that uncommon, it doesn't take a lot to upset the system setup software sadly. The only solution is to do a clean reinstall using the installation disk provided. Don't forget to install all the drivers too, these should either be on a disk provided or you can let Windows update install them for you.