I am a short name changing idiot and i don't know how to make my computergo

Hi

My girlfreind just bought a new macbook, and I inherited her old one - I used it all the time anyhow but since she was no longer using this macbook, i changed the email and stuff over to me - fine - all went well...

Then I decided i wanted to change the short name... and i did a net search and did something in my 'netinfo manager' utility something or other that someone suggested - and it changed my short name ... seemed fine...

for a day or two...

now i've figured out how to get all my stuff into my new short name account - all my itunes and garageband songs etc, and all seemed fine...

but

it seems i don't have administrator access anymore.... it's daylight savings and i discovered i can't change the clock...

and anyway - i tried changing the password with the osx install disk using my name (amy davidson and new password) - and it all seems fine... except this computer seems to have lost it's administrator and i don't know what to do - and mary's bought me a new version of logic express for this computer and i can't install it.... and etc etc etc.... just basic dilemas....

I was wondering if anyone else has gotten into this predicament before and can suggest a way that I can fix it.... i guess i could change back the short name to her name maybe - but then i'd need to wait another 2 hours while my garageband and itunes folders swap places again... and - oh gosh i'm stuck and need a hand...

it's late and i can't quite think strait now - and I realise now changing the 'shortname' is not recommended - but i jolly well did it before I knew that and now i'm stuck... and it took me two days to get my internet and email and garageband and itunes working since i did the name change and i don't want to waste another two days fluffing around with stuff I know nothing about....

any suggestions?