i had a problem that sounded exactly like this. the comptuer wouldn't respond to anything. wouldn't even turn on.

i took it to the store and the guy there wanted to charge me 300dollars to fix it. i hesitated and then the guy i usually deal with saw me and fixed the problem for naught, on the spot.

wish i could remember exactly what he said. but they just launched it from another computer and reset something. they said it had been a common problem

if it's going to cost you that much, it's worth getting a second opinion.