Sounds to me like a virus or malware at work here.
Then again, I hope you or something else didn't remap your drives.
To check your drive mapping just right-click on "My Computer" select "Manage" and then go to the "storage" section where you should then be able to expand the the "Disk Manager" and see what's attached and what's been partitioned. It's quite possible that your C: drive may have been changed. This can happen if you have an unruly thumb drive or some other kind of drive that wants to take over as the C: drive assignment. It's rare but it does happen. It's also not a big deal since drive mappings can easily be changed or changed back too. The only rule is that no two drives (partitions) can occupy the same drive letter. So in order to change your drive back to C: you might try right-clicking on say the X: partition (assuming that's what your original drive got changed to) and then select "change drive letters and paths" to change it back to C: . Of course, this assumes that something else isn't already occupying the C: drive assignment (which in your case isn't likely) cause like I said, there can only be one C: drive at a time.
Since I have no idea what you have in your computer, I think it's safe to assume you only have one physical hard drive. And in "Disk Manager" that one hard drive should be identified as "Disk 0" or it could be "Disk 1" or "Disk 2" etc. It's also likely that your one hard drive only has one partition which would hopefully be identified as the C: drive. However, it is possible to have other partitions with other drive mappings or even no mapping at all. But quite frankly, it sounds to me like your C: drive simply got changed to a different drive letter assignment which would explain why your browser refuses to browse to a drive that effectively isn't there. So like I said, just make sure you have your drives mapped correctly.
One last note you might want to be aware of: there are rare times when it is not desirable to assign the boot drive as the C: drive. However, it is typical that C: is mapped as the boot drive - in a single Windows only machine, that is.