Deleting files with only one operating system installed is no more or less difficult than with two. Just don't do it. Your windows partition will be obvious when you see it. If you really want to be sure then unmount it. You can also have Ubuntu mount it read only.
I would have a look here
If you're referring to when installing the second operating system then that's pretty simple as well. The installer will be very clear about how it's installing itself to the drive by giving you a few options. It will ask if you want to share the drive with windows, replace windows or do "something else" which involves manually setting up your partitions but I don't advise you doing that unless you're an experienced user and know what you're doing.
In the end, if you back up your files like we all should be doing then you're fine. Make sure to back up your personal files before installing Ubuntu and then do regular backups weekly. Important things like legal documents, files for work, family pictures, etc. should be backed up when changes are made.
To learn more about backing up your files, check this out.
Make sure to read the Ubuntu wiki section pertaining to installation as well. I would post a link but I don't have enough rep to post more than two.
Cheers!
more /etc/fstab
. – Rinzwind Aug 23 '14 at 17:42