None System Disk HP Compaq 615

Well this is the scenario, I have an HP Compaq 615 AMD 64 bit Athlon pc with a 160gig hard drive. The laptop is running vista basic and just failed to start for the second time stating that Non System Disk and tries to boot through the network. I tried restarting and testing the HP startup diagnostics and it says no Hard Disk installed. Then i shut it down and switched it on in like 20min then it starts up and asks for log in details just like that. So my question is whats next, should i reformat the hard disk, replace it or refer to my applications running which might be causing this. Luckily i do back up.