Why does Windows only detect one CPU on my dual core Intel system?
Unless your CPU is an Intel Pentium D Extreme Edition you must have Hyper-Threading disabled in the bios otherwise Windows XP Professional will only detect a single processing core.

Why doesn't my Western Digital SATA II hard drive detected in bios of my computer system?
You must apply a jumper on pins 5 and 6 on the hard drive, to enable SATA 150 M/B only operation.