Your old hard disk containe edht Operating Sytem for the Computer. (The softwate that makes it work.) When you removed the old hard disk you removed the Operating System as well. In order to get the PC working again, you need a recovery disk. If you don't have a floppy drive, you need to put the recovery CD into the CD drive, go into the BIOS setup (in your PCs manual) and boot from the CD Drive.
If you don't have a recovery CD then you need to get one. (PC maker or Microsoft.)
Alternatively you put your old drive back and install the new drive as the second hard disk and put all your vital data on it. Then when the first drive fails you haven't lost anything.
BTW, you don't say what you mean by 'playing up'.
Terry
May 2006