I just fixed a friend pc and because an error, its system hosting drive have stayed "E". is a way to change system drive letter?
Yes, it is possible in "Disk Management" MMC tool. Just open Control Panel > Administration > Disk Management or do Win+R
This answer explains the same for Windows 7, and while menus are called different, screenshots look identical.
Sounds like you possibly installed a 2nd drive to do an attempt at a repair.
You can change it. Since you said its a system drive, meaning it has the OS on it.
Turn the computer off, disconnect the power, remove any/all of the drives(including CD/DVDs) except the one that you want to be recognized as drive "C".
Turn the power on and it will boot up and recognize it as drive "C".
After a reboot and confirmation that the drive is in fact "C", shut down the system completely(unplug it so no residual electric is still on the motherboard).
To finish connect the next drive (hard drive/CD/DVD etc.) one at a time and repeat process.
This will work as I have done it successfully a couple of times with clients.
PS: If it is a hard drive with a couple of partitions make sure that the 2nd partition is labelled as any letter after the last drive that was available before. For example the last DVD drive was drive(G) then label the 2nd partition (H)
PSS: If the OS was installed directly on drive(E) then this will not work.