Disk Management is a very important part of Installation.
If you have 80 GB Space Hard Disk Drive and you installing OS On it.
Make Partitions like
C Drive :: 20480 MB (20GB)
D Drive :: 20480 MB (20GB)
Keep 40 GB space for future use.
If you are expert in computers and you want to do
dual booting, that time you will able to use that 40 GB space.
After Installation OF OS
You Can Just Right click on My computers
+ Select Manage
+ Select Disk Management
You will find 40 GB of Unpartitioned Space(Don't go for picture above it's just given for example)
Right click on unpartitioned space and make a logical drive.
(If you want to make partition in same OS )
You can use that unpartitioned space while installing another OS.
Your Friend Kedar