530s won't boot - tried many options

System froze when a program was loading (operating system is Vista). Had to do the "hold finger on power button" thing to shut it down. Since then when I try to start the computer it gets to the first screen with Dell logo, Bios version and F2/F12 instructions on and no further. Have tried both F2 and F12 and also F2/F12 with Operating System CD in and with Utilities/Drivers CD in but no response at all - just continues to sit at the Dell page. There are no beep codes and the activity light stays blue at all times, just like it did when the computer was working normally. The contents of the hard drive can be read when it is connected to another computer (USB connection in an external drive bay). Apart from picking up the BIOS revision (1.0.12) it doesn't want to know or do anything more.