Can not boot after plugging external USB drive, Need expert help to

Hi, I would appreciate if someone could help me fix the problem.

When I plugged in an external hard disc (from another laptop, cased) into my laptop via USB and tried to install Windows from a CD onto this external hard drive, I discovered that I could no longer boot Win XP on my laptop normally. I removed the external drive and tried to start my default system normally, it failed. The system won't detect the laptop's hard disc anymore reporting that there are problems with hard disc configuration. And so I can not boot any longer. I checked boot sequence in BIOS and it is set to boot from HDD first, then Win XP CD. HDD is in PM mode. CDROM in SM mode. The furthest I can get is to onto the windows installation screen, where the hard drive is not present.

Would you have any advise on how I can get the system up and running?
Thx