alright i will say the same thing that i said 20000 times but choobs never read other topics
1)Make your back ground
2)put your back ground (layer 2) below the original login screen (layer 1)
3)while choosing (layer 2) -that of your made background- erase the login box area .
4)drag (layer 2) -that of your made background- above (layer 1) -that of the original login screen- and here you go u got urself a login box with its same location / color / dimensions
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Easier way:
Use Magnetic Lasso tool on the login box(Magnify a little if u cant do it on normal resolution) copy and you are done