Booting to another hard drive

My computer yesterday was an XP machine with one hard drive. I wanted to test Windows 7 but I was afraid I would screw something up with my XP drive in the process. We'll call this Drive A.

Yesterday I bought a new hard drive, Drive B. I downloaded the Windows 7 ISO, got some drivers, and turned off my computer. I physically removed Drive A from the computer and put Drive B in. I started the computer and put the DVD in. Setup was fine, I had some trouble with getting the onboard NIC to work but I was able to overcome.

Now I needed to get back into my XP install to get some stuff done (play video games). I shut my computer down, pulled out Drive B, put Drive A back in and booted up. But I got a boot error message, and it looks like it's a Windows 7 Boot error message. How can this be if I removed the hard drive.

0xc000000e
The boot selection failed because a required device is inaccessible

It's almost as if my computer is trying to boot to Windows 7 even though I removed the hard drive containing the operating system. How do I get into my XP system? The drive is fine, I connected it as a slave and I can access the files on there. I'm guess Windows 7 makes some kind of change on the motherboard that forces it to boot to 7?

Thanks for any help!

sounds in deed very strange, there is no way windows can write a boot loader in a bios-based machine.

are you really sure, that this was a win7 message? AFAIK this message is the same in XP, so it might be a misunderstanding.

Did you put the drive to the same connector? (e.g. sata1 or ata1) as before?

another thing came in my mind: was or is a memory stick connected to your machine?

If you have really a win7 message, there might be a rare chance, that the win installer had put something to the stick, what means your computer try to boot from this stick.