Laptop Hard Drive Failure/Replacement

[KeithSECTalk]

Boot Device Not Found
Please install an operating system on your hard disk.
Hard Disk (3FO)

F2 System Diagnosis
For more information, please visit: www.hp.com/go/techcenter/startup

In the system diagnostics
F1 System info
F2 Start-up test
F3 Run-in test
F4 Hard disk test

F11 error log
ESC Exit

Luckily I have a Ultimate Electronics near me and they're selling everything 30-60% off, I think it's 40% off hard drives.

I opened up my laptop and found the hard drive and I have a few questions on replacing it.

Will my OS be ok? What will I need to do about that?

I'm thinking I might need to use some disc to help get the OS back, I burned a .iso disc for W7 but couldn't get it to do anything ATM which makes me think it's the hard drive. Going to go get a hard drive tomorrow and give it a go, hopefully it works out.