Tried replacing Jessica's video card and power supply with no luck. Then a check of the hardware database at work revealed the cause of my problem: Jessica's motherboard is almost two weeks beyond its warranty expiration date! Gadget had a look at the motherboard and found a couple of blown capacitors. These he replaced. Jessica then booted but when I got her home there were problems with the DVD burner and network card. Opened her up and carefully pushed everything into its position and now she's working again!