After replacing the old drive, did you put a new drive in? If so, then you'll need to partition, format and reinstall Windows before it will be recognized correctly.. Even a "used" hard drive as a replacement needs to be formatted, etc. or it won't be recognized correctly. It doesn't sound like you did that..
Hope this helps.
My motherboard went out last year and I took it too a local computer shop and they replaced the otherboard and I had them give me a larger hard drive.
The other day the same computer did a restart then windows did a "fatal error" message and shut down and restarted again and then it did a scan disk thing because it said that one or more of my hard drives may have problems or bad sectors or something and it scaned the disk. It said found errors and fixed them. Now when I start it up it says that I do not have a hard drive installed. I thought, "great my hard drive died" and I put in another hard drive and it said the same thing. I checked my cables and they are tight. I put in the WINDOWS XP PRO DVD/CD and it loads but then it says that there is no hard drive installed before it installs...
any suggestions on what this is?