The are many partitions still set as Active. Please set it as non Active. If you cant do it with Windows Disk Manager use Mini tool Partition. Leave only boot able partitions (like C:) set as active. Data partitions should be set as non Active.

Drive letters are assigned by the OS. For example, if you have another boot able disk you can have the same disk with a different letter or without one.

Lets do the basic.
Open a CMD window as administrator and type:
chkdsk C: /f
It will say the disk is in use and if you want to schedule to next start = yes
Reboot and pay attention on the results, specially bad clusters, bad blocks or bad sectors.

Back to windows, Open a CMD window as administrator and type:
sfc /scannow
report