If you are upgrading your PC, you probably won't lose your data on the D/E drives. The safest thing to do with it is when you go to install - disconnect the SATA cable to the D/E drive, that will leave only the C drive to install to. That way you don't accidentally install to the wrong drive. Some tips:
1) make sure your C drive is connected to the lowest numbered SATA port - it could be SATA0 or SATA1 depending on how they number the ports.
2) as mentioned, disconnect the secondary data drive for safety until you get your system up and running.