You said a tech support person said it was either the power supply or the motherboard. Has the PS been ruled out yet? Your motherboard will have the CPU and memory on it and either of these could have failed. Just what does it do and not do? The 845 designation on the board is the chipset and you would not be limited to this as there are new ones that support better RAM and faster CPUs but, being a brand name such as HP, you may not be able to find one that will fit properly. At three years of age, you have no warranty and might want to weigh the cost of repair against getting a new box. You might post back with some symptoms and get some decent clues here as to what might have failed.
I have an HP Pavilion 7966 that's less than 3 years old but recently died. A Tech Support friend from work has done some diagnostics and says it's either the power supply or the motherboard. I don't know how to buy something like a motherboard -- the pc is an Intel 845, Pentium 4 1.7 GHz/400. I've searched on "Intel 845" & am a bit overwhelmed by the choices that I'm seeing -- beyond that, I don't know what's really important that I should be looking for in a motherboard. How should I narrow my search? Thanks for any help you can provide --