hello when i first bought and played Fallout 3 when i looked at an item a code number would appear at the top of the screen.
This was the item code i was tolthis was what you put in using the cheat player.additem.
My query is that since reinstalling it on my brand new high spec computer this number does not appear now.
Is this a function you turn on or is this a mod i have missed off.
any constuctive answers welcomed.

You are talking about console commands, a developer tool left in the PC version. Comes by default in all versions of PC Fallout 3.

Something you'd need along with it is FOSE, a user made script utility for the game which enables the command "gbo", crucial in adding items you see in the game world. You see, the series of numbers you see when you click an object in console is just the Reference ID which handles nothing else than the object's existence in the game world. What you need is its Base ID, seen when using "gbo".
For example, if I drop a set of Enclave Power Armor on the floor and click it on the console it will produce a code something like: Enclave Power Armor (ff00xxxx) where the x's are random numbers and letters. I click the armor on the floor and type GBO. Hit Enter and console will produce the numbers: 0004443e. This is the Base ID and thus I will write the code: player.additem 4443e 1 to give myself Enclave Power Armor. And see what I did there? If the first numbers are zeros, you can leave them out.