Acer Aspire 7520, sudden boot problem

I just bought an Acer Aspire 7520 (exhibition piece, used - like new) and received it in the post yesterday morning. After a whole day of running fine, I used a friend's T-Mobile Web'n'Walk usb dongle later in the day and about halfway through install the system froze (except for cursor motion, I couldn't click on anything). I forced a restart, and after the initial boot screen it was left on an almost completely black screen with a white blinking underscore in the top left corner, even left for a while this didn't change.

I've tried restoring default settings in the f2 Boot Menu, leaving the system shut down over night, but nothing else so far as I don't want to damage it further.

Please help!

The hard drive data may be corrupted, or the hard drive may just be bad. Pull out the hard drive and see if you get a display of the initial post