Why did you have to reformat? Virus? In that case, a simple format will NOT remove it, you need to wipe the system drive. After that, perform a factory restore according to the manufacturer's instructions. Make sure you don't wipe the Restore partition, just the OS partition needs to be wiped.
Before you go with the wiping, you can try to do a factory restore by itself, but if it doesn't fix your boot up problem you might have to proceed with the wipe. Sometimes the factory restore procedure has a wiping step included.
If you are trying to perform a clean OS install from a different OS disk, then you have to perform the wipe before re-installing the OS again. That will ensure there are no viruses residing on the HDD prior to OS installation. Also, in this case, if the HDD is more than 5-6 years old, you might want to just replace the HDD and then perform a clean OS install.