Boot Sector Virus or Not?? Help Please!

Hello,
This past weekend I have run into a big problem. I recently was giving my sister a 6 gig HD, to replace her 2 gig storage drive. When I put the new HD in and booted up the computer. The computer wouldnt boot to windows. It stoped after it listed all the hardware. Just before the computer stoped, it looked in the a drive to find a boot record (to boot from floppy) then looked to the HD and said "Invalid System disk" like the HD was a floppy that is not bootable. So, I took a boot disk and check out the disks, and when I did I showed that all the data had been lost corrupted or something because nothing was showing up except a blank windows directory on the C drive. Now I am going to barrow nortons, to see if that can help, but Help me what is going on here. This thing happened instantaneously!!!

System:
AMD 650 T-Bird
512 PC100 RAM
40 Gig WD HD

THe six gig is a Western Digital as well.