When you creating your machine
1-Create New Machine Custom (Advance) -->
2-Customise your Hardware to your needs or leave default.-->
3-Select bottom option which is I will install OS later-->
4-Select your guest OS-->
5-Name Your OS and select the location-->
6-NOW SELECT "BIOS"-->
7-Select your Processor Config or leave default--->
8-Adjust your memory or leave default-->
9-Select your Network Type or leave default (NAT)-->
10-Leave I/O Controller Types as is default-->
11-Select your Disk Type or leave default-->
12-Select Disk or leave default-->
13-Specify your disk Capacity or leave default-->
14-Specify Disk File or leave default-->
15-Finish
16 Now go back to your newly machine right click and "Settings"-->
17 Go to "CD/DVD and select your CD Driver or ISO image --> Hit Ok or Apply
18-Now select your OS and click "Play" button and follow regular MS operating system installation and your preferences.