|So does it try to start up as soon as you plug it into the mains power or do you then hit the startup button to initiate bootup?|
You may have a powersupply problem since from your description this is an emachine you're talking about and they are notorious for crappy low end PSUs.
That is usually the best starting point when trying to diagnose these issues. Get a good quality known working PSU and see if that makes a difference.
Goin' Fishin' (Some day)