Usually that is because of a bad boot sector, if I remember correctly.
Flashing _ is looking for the HDD and the partition.
You can get into your BIOS on the motherboard, most likely the F2/setup key.
Try resetting all the options.
Make sure your HDD is the first boot option after resetting.
If that doesn't work, put the Windows OS disk into the disk drive and see if it can load the setup. Don't actually do the setup, I just want to see if you can do it.