Best place to start is your power supply - quite an easy change to make if your case is not too cramped.
I have one of these (
dabs.com - Dabs Value 400W MICRO ATX PSU (PSU400MICRO) as a replacement in one of my machines - its not the best and probably wont last (its done 18 months okay so far) but it is cheap. Besides if your machine is two years old in another 18 months will be about right to upgrade
You do need to check the power supply your replacing so that the new is at least as capable (i.e. dont replace a 400w powersupply with a 300w as it may not be enough - but okay to replace a 400w with a 500w).
Changing a power supply is simple, just unplug and unscrew the old and then put the new one in - you may want to write down which connections go where so you dont forget any but its hard to force something in the wrong way round.