Do you get any kind of error codes? What EXACTLY does the error window say when it happens? Do you get a blue screen at all?
First step would be to unplug all your hardware except for the needed stuff, like keyboard and mouse, if you still get it, than i would get into your box and remove any extra stuff you might have added, like NIC cards in the PCI slots, etc... try to narrow it down as much as possible.
Also, try right clicking on the "my computer icon, hit properties, and try to get into the device manager to see if there are any conflicts ( you'll see a little yellow exclamation point or question mark next to a device if there is one ).
Also, if you could give any details on what OS it is, service pack number, etc anything would be helpful in trying to figure this out for you. I know it's frusterating, but hang in there!
~ Gos