I bought a new hard drive and attached the power supply and ide cable and turned on the computer and it says operating system not found and i am unable to do anything else.
What do i do?

Also now after having unpluged the new drive i get the same warning and the only way to get past it is to enter the setup and load system defaults and save changes and restart, and even this doesn't work sometimes.

Hi Aalexmiller,
Welcome to PCHF

Try checking that the drives' jumpers are set correctly, "master" for the main drive with your O/S and "slave" for the new drive, also ideally if you have them on the same IDE cable put your main drive on the end plug and the other on the second plug. When your pc starts up, go into the BIOS and check that it is detecting the drives in the right order (master for the main O/S drive and slave for the other) and that the boot sequence is set correctly.
All the best

I tried doing what you said with no luck, exactly the same thing happens.
I did what i now have to do every time i start my computer which is enter the setup and restore system defaults and my aim was to see if i did this with both drives in if it would then work, however doing this comes up with this message:
disk read error occured
press ctl. alt. del. to restart
and the only way to get past it is to turn off my computer and unplug the new hard drive and start over again.
I am less worried about my new hard drive as i am with my old hard drive now, because the "operating system not found" message comes up everytime i load my computer.
Please help

Hmm, try unplugging your new drive completely, make sure the original is set to master and the bios set to boot from this first. If it still comes up with the operating system not found error try running the reovery from your operating system cd, or, remove your old drive put the new one in, install the O/S on the new drive and try to recover your data from the old one by installing it

In the bios everytime i load up my computer it seems to have the booting order set to the removable hard drives or something then the original hard drive automatically, but i presume there is nothing wrong with this because i have found out that all i need to do to get my pc to load up is to go into the setup every time and just to quit saving changes without actually changing anything and it seems to work, although it is a lot slower than normal loading.
So i suppose this means that when you say "try unplugging your new drive completely, make sure the original is set to master and the bios set to boot from this first" it still says operating system not found, so i should "try running the reovery from your operating system cd, or, remove your old drive put the new one in, install the O/S on the new drive and try to recover your data from the old one by installing it" ?
But how would i "install the O/S on the new drive" and "recover your data from the old one by installing it"?

The people who i bought the hard drive from said "The drives are already tested and partitioned NTFS" does this have any significance to my problem?