Why has my computer suddenly decided to boot off a secondary drive?

I loaded an old hard drive from a previous computer into my new one. It now tries to start Windows off the old drive not the new one. Is it to do with the drive names ie Drive C and Drive E? I think it's changed around. How can I change this so it just boots from the new version? I have to manually do it at the start up screen at the moment.Why has my computer suddenly decided to boot off a secondary drive?
At the start screen you press del and go into bios. Its different for the majority of machines but should be boot sequence or something like that change it so the other drive is set to come first. At the end save and exit. will restart computer. Make sure you dont change anything else if u think u made a mistake exit without saving and try again.Why has my computer suddenly decided to boot off a secondary drive?
The drive you fitted from your old computer should have been set to be a SLAVE you do this with the pin links

You can if you are running Vista or W7 change the drive path letters around via the computer management / disc management option
All you need to do is change the boot order in the BIOS.

Instructions here: http://pcsupport.about.com/od/fixtheprob



Alternatively, you might need to either edit the boot.ini (XP) or use bootmgr (Win 7)