My current OS is Windows 7. I have 1 HDD (1 TB). It has a drive C: (100GB), a drive D: (~800GB), and unallocated disk space (100 GB) I intend to use for Fedora.
Can I dual boot Fedora 30 with Windows 7 (To use both of these OS)?
Yes, you can! (as The Man said).
When you install Fedora (and many other modern distros), you are given the choice to take then entire disk (which you don't want), to use available unused space (excellent choice), or to resize an existing partition (such as D: or C:) and give some of that space to Linux (not necessary from your description).
Each Linux kernel only takes around 10 MB, but new kernels may be worth adding for your consideration. Other minimum space requirements include:
Directory Min. size
/ 250 MB (where your kernels live)
/usr 250 MB (avoid placing this on a separate partition) /tmp 50 MB
/var 384 MB
/home 100 MB (this is where your data will live) /boot 250 MB
Having a separate swap partition is passe, as ever since the 2.4 kernels, swap files run just as fast.
Sure you can,
boot from a live disk or USB drive and make sure to install you new system on the targeted disk. It's a good idea to make a backup of your important data just in case.