Now I am running windows 8.1 on my laptop.I have 3 disks(C,D,E).Windows 8.1 files are in C disk.Now I want to replace windows 8.1 with ubuntu 17.10.1. How can I keep my D & E safe and install ubuntu without losing any data.how the disk partition should be?
It sounds like your hard drive is partitioned into three partitions. If so, it would be much safer to install Ubuntu on a new Hard Drive, and then drag and drop everything from D and E over to the new drive.
If you want to take chances and install Ubuntu onto the partition that contained the "C" drive, then make sure you can identify that partition by its size. Click on File Explorer in Windows and then right click on the C drive and select "properties". Do this for each of the drives and get the size of each. While installing Ubuntu make sure you select custom install option or "advanced" and then select the partition that matches the C drive size.
If D and E are separate drives, remove them and install Ubuntu on the internal hard drive, and then reinsert the other drives.