Car won't keep charge

I inherited my boyfriend's car which had
been sitting dead for the last six months. I
replaced the battery and the alternator lay
week and everything seemed to be fine until
yesterday. I accidentally left one of the
interior lights on and it died. So some
friends went to go jump it. They got out
running but didn't leave out running long
enough after so it didn't charge. I went out,
jumped it, and ran it on the interstate for
almost an hour and then went home. Tried
too start it again and it won't even turn over:  
no lights, no nothing. Took the battery in to
be tested and it says that it's good. So now
at a loss of what to look for.