wont scan high resolution

I  bought a dell v505 all in one usbed it to dell dimension 4700 2g ram 250 hard , when i try to scan resolutions above 300 dpi it sends message comp does not have enough disk space even though system says i have 200 gigs free. it also enlarges photo 4x on its own , a 4 x 6 photo at 300 dpi is taking 7.7 mb and fills 19 inch wide screen , same photo from old scanner  640 kb barely quarter of screen . any ideas?  i'm MAD & CRAZY