Obviously depends on the laptop but I would expect a hinge or connector problem simply to blank the screen rather than shutdown / standby the machine.
The lid close switch is certainly a possibility though.
You should be able to change the options so this simply turms the screen off rather than initiating a shutdown or standby.
As Keith said antivirus programs don't catch malware.
I agree 100% the combination of Spybot, Spysweeper and Ad Aware find and clear out most bugs, you really do need to run them all.
Also do you use Internet Explorer?
If so to avoid potential & future hassles download & use Firefox instead.
Limit the number of Plug-ins to those you really need too.
As Firefox is far less integrated with Windows it gives attackers a much harder time.