Windows wont boot without flash Drive USB Drive CD
This problem cause by bootloader, a program that load your operating system. So, if your hard disk (internal) does not have the bootloader it wont boot to windows because no program will load your operating system (Windows XP/7/8/10 etc). In one case (Asus Notebook) the bootloader not installed in C partition (the windows partiton he use to install windows) but installed on the USB disk. So, without the USB disk he cant boot to windows.
So here how to fix it:
First of all Insert you installation Disk (either DVD or USB Stick)
In Windows 8:
When installation windows appear Click Next - click Repair your computer
click Troubleshoot
click Advanced options
choose Command Prompt
Then type: (press enter to execute the command and do it one by one)
bootrec / fixmbr
bootrec /fixboot
Windows Vista/7:
It is the same method as Windows 8 you need to access the command prompt. to do this insert you installation Disk and select repair. Then select keyboard - Use recovery tools - Command prompt
The type the following command one by one and press enter after each line.
bootrec / fixmbr
bootrec /fixboot
You can also use Bootrec.exe by microsoft instead. I never try it and it is a lot to explain how to use this tools. so I just write the easiest method to fix this =p
Go to link download