Is this possible?

As friend asked me to look at his computer. It was not booting correctly and was showing a Boot.ini error. I was unable to enter safe mode and he did not have a system restore disk. I ended up slaving the "bad" HDD to a good system to fix the Boot.ini file this just led to a Hal.dll error when the "bad" HDD was reinstalled in the system.

Rather than waste time tracking things down I removed a HDD from a fully functioning system and placed it in the non functioning system. The nonfunctioning system will now only boot into safe mode and will not start normally.

The question is can you simply swap out hard drives from one system to another or are there other things that need to be done?

Thanks.