Use Disk Management and you can change drive letters around.
BUT - If your Windows drive is F - then all you installed settings and programs know that - and if you tried to change it to C they would all be totally confused.
So - basically you're now stuck with it!
If you install another drive, it will normally select the first unused drive letter - which in your case is C - though that you can safely alter.
How it happened in the first place is impossible to answer at this late stage.