My laptop restarts whenever I try to start Windows without my external HDD connected - after I log in an error message pops up (one of those 0x... errors) but it reboots before I can read what it says. I've tried changing the boot order in the BIOS but all that did was make it restart a few moments after the login screen loads. If I wait, it restarts, and if I log in, a message saying something about the Security Accounts Manager pops up and it reboots again.
Windows 7 works fine when the HDD is connected, and Crunchbang Linux (dual-boot) works fine in either situation.
The only things I did that might have messed something up was install some Windows updates and install Autodesk Inventor to the external hard drive.