Ah... yeah, that'll probably be it.
The BIOS can see it, yes? If not, the problem is not missing boot code.
Assuming the BIOS recognises it just fine, I'm guessing the disk simply doesn't have an appropriate Master Boot Record on it. Take out your other hard disk and set up the new one as Primary Master, yadda yadda yadda. Dig out your Windows CD and get to the Recovery Console. Log in and use 'FIXMBR'. Say 'yes' to anything it asks; you have a backup of the disk so it isn't going to do anything irreversible.
Now use 'BOOTCFG /REBUILD'.
(I'm not 100% sure about this syntax (I haven't used the RC in a while) so try 'HELP BOOTCFG' first. The option will be something like /REBUILD or /RESCAN).
It should detect your boot partition and set up the Windows boot manager correctly. Reboot when it's done and everything should be fine.
Note that 'should' is the operative word. I have hit so many problems with the Windows boot manager that's just not funny any more. If it still doesn't work you might try 'FIXBOOT C:' at the Recovery Console, although the boot sector should've been copied as part of the partition.
If you can't even log into the Recovery Console, I'd suggest blanking the partition (keep it the same size as the one on your original hard disk), installing Windows, then recopying the partition over from the original disk. Then resize and whatever.