How could I install Boot Camp drivers to my Windows partition?
Probably your Boot Camp partition is hosed and couldn't be removed simply.
So i would try to reinstall Windows 7 with your pre-made (Microsoft's bootable USB creation tool!) thumb drive.

Attach the Windows 7 installer thumb drive to your Windows server,  format it if necessary (FAT32) and create the bootable Win7 install thumb drive again with the Microsoft tool.

Download BootCamp5.1.5621 on your server

Extract BootCamp5.1.5621.zip

Copy the extracted files and folders ($WinPEDriver$, AutoUnattend.xml, BootCamp etc.) to the root of the thumb drive.

The file/folder structure in the root of your thumb drive should look like this:


Detach the thumb drive from your server

Attach the thumb drive to your Mac

Reboot your Mac and immediately hold the alt/option button until you see the boot menu

Choose your thumb drive as boot volume

Try to reinstall Windows 7. You might have to reformat your existing Windows partition with the disk management tool of the Windows 7 Installer (don't delete or resize it!).


If that doesn't work reboot to Mac OS X, start Boot Camp Assistant and try to remove Windows 7 completely.
If the removal fails check several answers here to remove your Boot Camp partition manually:
remove-windows-7
i-messed-up-the-fusion-drive (both require a Time Machine backup)
or some other answers here.
Before doing that post a comment and i will give advice depending on your partition layout.
Start Boot Camp Assistant from scratch to install Windows 7.