PC hangs with striped screen and then three long beeps on start-up

re-seat your RAM module(s)

"This wouldn't explain the beeps, but test the screen quickly on a different input to rule out any problems with that.

Next, the beeps! if the screen is fine and you are getting that output, it would mean that the motherboard is having problems. First step would be to remove everything, and just leave in power, one RAM (if you had more), nothing external apart from the screen and no hard drives. We are not trying to boot up, we are just trying to get past the BIOS.

If this works, gradually put items back one by one until you get another failure.

If it does not work, and you are still getting beeps, you most likely have a dead board. Check for capacitors that have blown

The top should be almost flat (with slight indents out embossed sections depending on specification... look at the middle one) but you do not want to see any big bumps such as the first one or any leakage as the last one.

If your pc is of the late P4 generation, there were MANY MANY boards that had this problem which lead to a lot of random failures as you are experiencing now."