New battery needed!
Is the battery you have now one of those "pitwork" ones? If so they normally die pretty quickly. I know during the first few weeks of getting my car I had similar problems.
Also another SAU member's GTR left it for a week and it wouldnt started (Also had a pitwork battery - I think they are std. nissan ones from japan), tried jumper leads, pushing it no go. Changed the battery over to a new one and it worked straight off.
There is a group buy on the FS from wrxhoon who has those dry gell type batteries for $150 I think. Check it out.