How do I get rid of this old hard drive in my machine?
When you pop in your windows 7 dvd and enter recovery mode.
You will want to run the following cmd on the cli:
fdisk /mbr
That will fix the 'master boot record' of the current drive and will make it bootable according to windows standards.
Chances are that the boot sector is on the drive. Pop that drive out, pop in your windows 7 DVD or recovery disk, and select system recovery, and the startup repair mode to fix it. If that works, then you can nuke the old hard drive
My first answer would be a bios setting. Go into the Bios and change the boot options. It is most likely one of the below settings.
It is set to IDE or mixed mode and thinks there are no drives anymore since it can't find the IDE drive. If this is the case you need to change it to SATA mode or equivalent setting in your BIOS.
You may need to set the BIOS to not look for whichever slot it was plugged into.
Perhaps it is just your boot order. Sometimes pulling drives out can mess that up and it is now looking to boot from something completely wrong.
If it isn't a BIOS setting, then doing the fix mbr should work.