Windows 7 - Choosing the default OS or removing an option...

I have Windows 7 installed on my computer. I'm helping a friend fix his computer. He has a netbook with no DVD drive and I couldn't find my external DVD drive. I plugged his hard drive into my computer to access it. I successfully installed windows to his hard drive form my computer. I then removed the drive and will place it back in his computer soon.

Problem is... now when my computer boots it tries to boot from the Windows 7 installation I did to his hard drive. I get an error saying that the drive has been removed or is not accessible or whatever. Obviously that's because I removed it.

Question is... can I change the default OS to boot from? I have 2 Windows 7 options (mine and the installation I just did on his). Can I remove the option to boot from the recent installation I did on his hard drive?

Many thanks in advance!!

Yes, although if your system shows his HDD to boot from, I'm guessing the Windows 7 install on that HDD doesn't have boot info and you're going to run into problems when you put it into his machine - and without a DVD to boot from, you aren't going to be able to run fixmbr or run a boot repair from the setup either to fix it. I'd suggest (strongly) putting the Windows 7 DVD on a USB key and installing with the HDD in his netbook.

As to removing the option, yes. There's a tool you can run from a command prompt called bcdedit which allows you to modify the boot options for your system. You would run bcdedit /enum to get a list of boot options on your system, and then bcdedit /delete {<insert GUID here of boot object from list>} to remove a boot option.