You said that you're currently running Windows 7 and have it installed on partition C. Partition D is empty, and you want to install RHEL6 on it without affecting Windows 7 (i.e., dual booting Windows 7 & Red Hat6).
First of all, log into Windows 7 and remove the D partition from disk management. Go to the run dialog box and type diskmgmt.msc.
Now, check free space. You can use the shrink option in Windows 7 to decrease your main partition (the C drive). Right-click on the C, then shrink the volume. After that you have to restart your system because you changed your main partition configuration.
Then, boot from the RHEL6 DVD and install RHEL6 normally. Now during the installation, when RHEL6 asks for type of installation, choose create custom layout.
My partiton layout is:
Windows
- C partition is 28 GB
- D partition is 128 GB
Linux
/ partition is 26 GB
/boot partition is 500 MB
/home partition is 16 GB
/tmp partition is 4 GB
/usr partition is 6 GB
/var partition is 14 GB
/usr/local partition is 4.5 GB
/opt partition is 11 GB
swap partition is twice the size as your RAM
/yoo partition is 16 GB, vfat filesystem
If you're using two HDDs, then the installation is different, so I recommend you to install on one HDD only.