In some companies, you can find that access to the C-drive of a Windows computer has been blocked. How do they do this?

You can easily do this by creating an entry in your domain policy. The path to the policy is
User Configuration\Administrative Templates\Windows Components\Windows Explorer
In that location, two policy-settings are available:

• Hide these specified drives in My Computer: When you enable this setting, the effect will only be that the selected drives will not be visible in Windows Explorer, My Computer of the standard Open dialog boxes.
• Prevent access to these drives from My Computer: If you enable this setting, users can browse the directory structure of the selected drives in My Computer or Windows Explorer, but they cannot open folders and access the contents. Also, they cannot use the Run dialog box or the Map Network Drive dialog box to view the directories on these drives.

How can you bypass this?
Such a policy only prevent the Windows Explorer from browsing into the C-drive. The DOS prompt or another file browser will not be stopped from peeking into the blocked folder / drive.