Linux is a form of unix, and the old joke goes that “unix is very user friendly, it’s just particular about who its friends are.” Linux has gotten a lot more user friendly over the years, but its still a bit away from the point and click windows stuff.
I always use slackware instead of redhat (only because I’ve been using slackware since it was just about the only distribution you could get, long before redhat came along). No matter what distribution you use, you end up doing basically the same things.
The least risky thing to do with your D drive is save all your stuff somewhere, wipe the disk, and repartition it, format half of it, then put your windows stuff back on it. Just leave the rest of the disk unpartitioned until you put linux on it. Linux really needs a root and a swap partition. Redhat may want to set up other partitions for you. Do whatever is easiest to do in redhat, but all you really need is a main linux partition and a swap partition. The swap partition should be at least double your physical ram size, and probably doesn’t need to be much bigger than that. The main linux partition can take up the rest of the free space on the disk.
You can try to repartition your drive without saving everything off of it. This can be a little risky but most of the time it works.
In slackware and debian you create boot disks, then once you boot from these you install from the CD. I imagine redhat is similar. Then once linux is all installed you get to configure it, and this is where redhat differs from some of the other versions. It has a lot of its own configuration tools, and since I don’t use redhat I can’t help you any there.
Almost every linux system I’ve had has been a dual boot system. I’ve only ever installed one that wasn’t. I originally only used LILO, then once LILO completely wacked out my MBR and made the machine unbootable. Fortunately this was back in the days of windows 95 and simply booting from a floppy and doing the old fdisk/mbr fixed it. After that I was a little shy of LILO, and I always used LOADLIN instead. I would usually set it up so that windows had a Linux icon, which was just a shortcut to LOADLIN with the “run in ms-dos” mode switch turned on (forced the computer to reboot into dos, then loaded linux from there).
The last linux system I set up was win2000/linux, and since win2000 doesn’t have dos I couldn’t use loadlin, so I decided to be brave and see how well LILO worked these days. On my one system (not exactly a broad test) it worked fine.
As long as you pay close attention to what drive and partition you are shoving things into, I don’t think you will have much of a problem.
Make sure all of your hardware is supported by linux before you start.