have you already reinstalled windows? if not you need to install it in your new hdd.

nanapapa

I have the exact same situation with an Optiplex GX-260 and just keep doing the F12 trick, since I (too) do not know another way to get it to run (which seems fine).

Thanks for the replies, but I found the problem. I did install XP on the new drive. The problem was, NONE of the boot devices in the list in CMOS was enabled. Since none were, they all looked alike, and I didn't know (this machine belongs to a friend, I had never used it before). Today, I clicked on the space bar to see what it did, and saw the color change and an arrow appear to the left. I realized that none had been enabled, so I enabled the CD (first in list), then the hard drive (second). Now it boots up fine. To get to CMOS, hit F2 instead of F12, and do an Alt-P to get to the page (second I think) that allows you to change the order of the boot devices.

Good luck.