Did you FDISK your machine when you reloaded your system, or just reformat? If you didn't delete the partition using FDISK you could still have a virus in the MBR , or just a corrupt MBR.
Run through this
checklist and check all your BIOS settings. If that doesn't work, go back and start again from scratch using FDISK to delete the present partitions first.
You can then update your BIOS and drivers from
here with the latest versions.