Turn on and go directly into BIOS Setup - watch the screen for the right key to push because I'm not sure it is "Del" on a Dell. Go to where you set the Boot Priority Sequence. Make sure it is set to try your optical drive first (if that's your choice), then the new HDD second, and NO other options allowed after that. Save and Exit. See if it boots.
Within BIOS Setup, does your system recognize the old drive properly? If not, it probably has a significant hardware issue that makes it really hard to recover anything.
If the BIOS is set NOT to try using the old drive but it still will not boot with that drive installed, the drive probably is causing major errors as it is first checked in the boot process, and Windows or the BIOS is stuck re-trying it to get past the errors. In that case, get a good disk diagnostics package like Seagate's Seatools or WD's Data Lifegard. Get the version that you burn to your own bootable CD. You boot from it and run independent of Windows to do all your testing work on the suspect drive. That can tell you what kinds of trouble it has.
Thanks, Paperdoc. I'll follow your recommendation this PM and share results; I'm @ my day job, now.
Something else to add to the mix: a family member suggested loading Linux's Ubuntu OS to see if the old HDD would boot and operate, and it did; I could see all my files. From this, I'm assuming the old HDD is, at least, mechanically OK.
OK, that's good info. It says there is really no hardware problem with the old HDD. So do the Boot Priority setting thing first. With that adjusted you may have your problem solved. But if Windows still has trouble seeing the HDD, there is a software issue to resolve, not hardware.
One other thing to check. When in BIOS Setup, look where the SATA ports are Enabled etc., and check the way the port mode is set for your drives. I am betting your XP install did NOT include adding in special SATA or AHCI drivers, so you are depending on a BIOS port mode setting to be IDE (or PATA) Emulation so Win XP can use the device. (This mode has the BIOS fool Windows into thinking the actual SATA drive is simply an older IDE device that it understands fully.) Make sure the ports modes are set to IDE emulation.