need help with vaio vgn-fj170/b HELP???

HI all,

I just bought a working Vaio fj170/b (minus the hard drive) because of the fact that my old one's mobo was fried. I threw in the memory from my old one, threw in the old hard drive & booted...it worked, booted Windows XP up but when it hit the desktop it 'recognized' new hardware (different DVD drive) and froze... stopped reading the HD, stopped everything. couldn't move the pointer, nothing. Thought nothing of it, waited a few minutes & turned it off. Restarted. NOTHING. The power light turns on, it tries reading from the DVD & stops. Doesn't try reading the HD. I then thought nothing of it, I also have a fresh Vista install disc. Thought I'd try that. Booted, nothing. Won't read it, though it acts like it's reading from the DVD drive. Oh, and all the while, I have no display either... It's just not doing anything other than powering on..not reading from the HD, and pseudo-reading the DVD. HELP?!? I've tried switching the memory cards around, one @ a time, different slots, etc...no avail. anyone have any ideas on what to try next?? or what could be wrong with this??