When you install a new hard drive (even if it used), your mobo (the same as mine) makes that disk the priority one disk, thus it becomes the boot disk! So what you have to do is go into your BIOS and change the hard disk priority and make the hard disk that has the operation system be the primary disk and boot disk. For some reason every time you install a new drive the BIOS of that mobo makes it the primary boot disk even tough it me be empty.
Asus P4P800 SE
Intel P4 3.2 Gig 1024 Meg PC3200 cas latency 2.5
RAID 0 2-36.7 Gig SATA WD 10,000 RPM
200 Gig WD ATI ALL-IN-WONDER 9600XT
6500 Kb bandwith Internet Connection