Changing letter of drive C:

[KSoD]

Doing that will not solve you issue. It will first change the drive letter from with in that 2nd OS not the primary one you are trying to change. IT will have no effect on that system at all. In fact you could damage it and make it un-usable.

As stated a format will be the only way to get it to be the C:\ drive. Sorry. I know that is not the answer people want but it is the only answer that is true. Installing a 2nd OS will not affect the first ones drive lettering.

 

[neogenesis]

This has happened to me and I was able to resolve it. All I did was add the conflicted hdd to another running computer with windows. From here you can see both partitions and if it hasn't been formated, format it so both are active. Now through computer management I set both drives to a letter but assigning the partition I wanted "C:" to any letter that comes before the second partition letter.

Now stick the hdd into the original computer and windows will detect the partition with the OS from boot and automatically assign the drive to "C:".

 

[KSoD]

But yet again your involves a format and reinstall. Which is what we are suggesting.

 

[neogenesis]

I'm not saying reformating. This can be done without the lost of any information. My two partitions switched letter drives due to Partition Magic and I did my method and booted up fine without reinstalling windows.

 

[KSoD]

This has happened to me and I was able to resolve it. All I did was add the conflicted hdd to another running computer with windows. From here you can see both partitions and if it hasn't been formated, format it so both are active. Now through computer management I set both drives to a letter but assigning the partition I wanted "C:" to any letter that comes before the second partition letter.

Now stick the hdd into the original computer and windows will detect the partition with the OS from boot and automatically assign the drive to "C:".

Right there. You say format. Which means to wipe the drive of all info. Data lose.

Unless you mean change the properties of the drive to make it a active system drive. Which you can do with GParted or Acronis Disk Director as well.