However this is not the most pressing issue at the moment as now drive C: is that full it doesn't have any room left to work so to speak.
You should have been getting something like "Disk Full" errors showing at some point.
The drive that does not have a drive letter does indeed appear to be a recovery partition so your best option would be to do a back to factory reinstall via the tools in Start/ All Programs or using any disks that you may have made.
Doing so will then enlarge C: back to its original size by removing the unallocated partition and will leave the recovery partition intact.
Please tell us the make and model of your PC also.