When you open Windows Explorer (by choosing the Window key and "E" simultaneously or by choosing Start, all Programs, Accessories, Windows Explorer), you can change the directory that appears by default.

If you choose Start, all Programs, Accessories, and then right-click on Windows Explorer and choose Properties, you can modify the "Target" directory.

To go to your C: drive, type simply C:\ in the Target box and choose OK. You can also enter a shortcut key on this screen, telling Windows the character or combination of characters you want to type to automatically open Windows Explorer. You can even change the icon or specify that you always want Explorer to open up in full-screen mode.