Computer attempting to boot from external hard drive

The topic kinda says it all. Every time I reboot my computer (or just boot it, but my computer doesn't usually stay off long, lol) I have to unplug my external hard drive. If I don't, the computer tries to boot from the external, and I get a "no bootable device" error message. The strange thing is that the is no option to boot from USB in the bios, and the first boot option is to boot from the normal hard drive. It's getting really annoying… Anyone have any idea what it's doing or how to fix it? The OS is Windows XP, though I don't think that matters. The computer itself is third hand, and has had so many parts replaced it basically a custom job from what I understand. I'm not very hardware savvy, so I'm not sure what would be relevant information… Thanks in advance.

There is another setting that works in conjunction with listing the order of boot devices in the bios, try looking under Drive Priorities, or similar wording, within that it lists your hard drives, including USB ones (on mine anyway), within this option you tell the system which of the hard drives is to be the 1st drive to use, make sure your OS drive is listed as the 1st drive in that list, then go to the Boot list, and make sure the hard drive is the 1st option (it's faster), followed by the optical drive as 2nd, TBH any further boot options I simply set to disable, then it wont waste time looking for what isn't there, see if that helps. One point to note though, I don't know enough to say why, or how to know when, but I have noticed on systems I mess with, that if a drive is added\removed\re-added to the system, it can change the Drive Priorities again, so after messing with drives, you find it wont boot from the correct device again, recheck that the OS drive is listed as the 1st one under the Drive Priorities again, I don't know why this happens, or how to stop it happening, or even if it's possible, never really looked into it TBH, but I have seen this happen a lot, and am guessing it may be the same as your looking at.