If I understood you correctly, you may find this link and this link useful.
Like in Windows, in Ubuntu you can split the drive in multiple partitions.
When installing Ubuntu, you can choose to install the system in one partition
(the / directory) and your user files in another partition (the /home directory).
Then, when you reinstall the system, you can keep your user files intact (but you
should always do a backup before!).
Consult the two links above for more information.
Also, Ubuntu doesn't use drive letters like C:.
Instead, everything is inside the root directory (/), and other partitions are
"mounted" into some folder inside it (like /home).
So, / can be in one partition while /home, even though it's inside /, can be in another partition (even in another computer).
During install, choose "Something else" to setup the partitions manually (be careful with it, though).