I was trying to format a hard drive to swap into my laptop. Now my OS wont start.?

Actually you can call HP (who sell Compaqs) and have them send you a set of recovery discs, although they may charge you a small fee. You should have created them when you first got the computer. You cannot just "back up" another computer and use that drive to run another computer, because it "backing up" does not create a bootable hard drive. There are programs you could have used to migrate the old drive to the new one, but it is too late for that now. Your best bet is to call the dealer where you got the computer.