My 7410 All-in-One printer stopped in a blue screen with 00000020

0000020 is a BIOS error code for WiFi communication error. (Took me HOURS to find that in 2008!)
1. Power down the printer - disconnect the power supply 20 seconds. Restart it while pressing the ON along with OK both for 3-5 seconds. This will recover a failed BIOS. However, if the WiFi really is dead (and it IS the weakest link in the printer), you may get the error again.
2. Power off the printer (again). Disconnect the WiFi card (back panel, look for a 6-7 wire cable wrapped in plastic tape). If the printer restarts, you have your answer... but not the solution!
3. HP7400-series WiFi cards are rare as hens-teeth these days. Mine failed a 2d time; I use the cat5e connector and just leave the WiFi disconnected.