Well, I just got this gaming pc today. The owner assured that it worked and was ready to go, it just needed the OS.
I tried installing a copy of Windows XP Pro and it loads up the disc but when it gets to the part where it saves stuff to the hard disk/drive, I get a message saying that the Hard Disk isn't detected/isn't on.
I already opened up the case and I can see that the hard drive is there and it looks like it's connected properly but it doesn't seem to detect it. Any help?

Some info on the pc:
CPU: Pentium "D" Dual Core 3.2 GHz CPU
Gigabyte GA-965P-S3 Motherboard
Graphics Card: x1950xtx w/512MB GDDR4
2GB (2048 MB) DDR2 Main Memory
300 GB 7200 rpm SATA Hard Drive
Pioneer DVD/CD Burner Drive
3.5" FDD
500 Watt Power Supply

See if the HDD is being recognized by the BIOS. If it is, could it be that you're not pressing F6 during the XP install & loading the SATA drivers?

The bios must detect the drive.

If it is a SATA-II drive (a.k.a. SATA-2; capable of 300mb/sec burst speeds) and it wasn't in the computer when you bought it, if your mboard has SATA support but not SATA-II support, some mboard chipsets that recognize SATA drives cannot recognize SATA-II drives and run them in SATA mode unless you can, and do, install a jumper on the drive to limit it to SATA mode (150mb/sec max burst speed).

If the drive is SATA, most computer bioses have two or three modes the SATA drive can run in - IDE compatible mode which is often the default, or SATA mode, or if the SATA controller is RAID compatible, SATA RAID mode (labelling varies - see your mboard manual and bios settings descriptions in that).
If that setting is in IDE compatible mode, Windows Setup will find the drive without you having to load SATA controller drivers at the beggining of loading files from the CD, but if the mode is SATA mode or SATA RAID mode, you have to do as jam suggests.

Ah, well I got it working now, thanks for the help you guys!