In A Boot Up Loop Booting Up?

It's a good thing I have another computer to use...otherwise, I don't know how I would be able to ask these questions.My other machine is an HP a730n, 1 Gb RAM, 3 GHz, running with AVG anti-virus, all Windows updates in place.The machine was on overnight. Two programs were running (photoeditor, word processor).This machine also has two external drives connected to it, through USB ports.When I turned on my monitor, there was an error message displayed, but I didn't write it down. I clicked okay, and then the machine proceeded to shut down, and now it is going through an endless loop of trying to start windows.It initially gives me a screen with the options of safe mode, safe mode with networking, or command prompt, letting it start with the last known working configuration, or start windows normally. I've tried the start windows normally several times, to no avail.In safe mode, I clicked on Administrator, which then said it was starting windows up, but I then got a window that said "winlogon.exe application error. The instruction at "0x7498aaba" referenced memory at "0x00000008." The memory could not be read. Click cancel to debug, and OK to close.I also tried the last known good configuration, with the endless loop continuing.