As scorpion99 says, check that there are no external devices attached (I sometimes forget this). But it seems to me that your real C:\ partition is not mounted, and because of that, it's letter might be assigned to another device.
You can try to backup your files using an Ubuntu Live CD. Instructions here (screenshots are a bit dated): http://www.howtogeek.com/howto.....-computer/
You can download the ISO image of Ubuntu Live CD from http://www.ubuntu.com/download/desktop . Select 12.04 (64-bit version), and then on the next page go to the bottom and select "Not now, take me to the download". If you have any questions or trouble with this just ask.
To fix your problem you could try to follow these instructions to access a Command Prompt: http://www.howtogeek.com/forum.....ost-318915
I suspect you won't be able to backup your files as it is shown there. But from that Command Prompt, try using these commands: