I can't find the exact model but I'm finding many folk reporting "it's going to sleep" because the monitor shows such a message. What's really happening is the laptop shut down and the monitor shows a "going to sleep" message so let's move on.
Given the GMA I have to guess the age but let's check if you are using the canned air to clean every few months.
I recently hooked up an external monitor to my ASUS laptop (running Windows 7) and it's now going to sleep at times as I'm working on Word, viewing a video, or without me doing anything. I've changed the power settings, had the PC checked out at the shop, switched out the monitor and the power chord, and it will still go to sleep. It usually does it with several programs running (Quickbooks, Firefox, Word, Outlook, Itunes) at a time and late at night after it's been running all day. It often does it when I'm streaming video. I added RAM to it and that didn't help either. (It never goes to sleep like this if I'm using the laptop's monitor only.) Here are its specs:
The chip type: Mobile Intel (R) 4 series Express Chipset family
Adapter string: Mobile Intel (R) GMA 4500 M
Total available graphics memory: 1326 MB
Dedicated Video memory: 64 MB
System video memory: 0 MB
Shared system memory: 1262 MB
Last night just before it went to sleep I noticed I could hear the fan running and it sounded like it was working pretty hard.
Any help would be greatly appreciated.