Do you have a xp cd as you'll need it to sort this out.

I am not sure exactly what you did.  Are you saying you copied all the Windows XP files from a zip file to the external drive,  moe that drive to internal and tried to boot from it?

I Already installed the hard drive into the Dell Optiplex 745 and tried to start the computer. All i get is a Black Screen with NTLDR is missing. What can I do to fix It?

How did you get the OS on the disk?  You can't just copy it, on to the disk, you must use the windoes install procedures

If i understood it well, i think you only copied files and the boot files are not copied or they have problem. You should install windows to work not copy it. Also you can use softwares like Norton ghost for restoring your windows well,  but copying it will not work.