Lets start with the basics.
1)Has it had a drop of a liquid spill?
2)Has the memory been upgraded?
3)Has HDD been upgraded?
4)Has any other type of software been install other then what it came with?
5)Has Windows been install through Boot Camp?
Have you try Repair Permissions like machead3 said?
1)if drop it could of loosen something, damage the heat-sink or even the logic-board itself. If a liquid spill the spill is starting to eat away on the logic-board; Clean the logic-board asap before it become a total failure.
2)The memory max is 4gb but you could put 6gb, but you pull the a aftermarket one they do fail. Your better doing the max the apple say of 4gb.
3)If you up grade HDD you might have a defected one.
4)Some software's do share folders, but when folder are missing the system could freeze but rarely. Check for updates and if you could reinstall the software. Better yet you should do a full reinstall of MB.
5)If Windows been installed then you should by now Windows operation system could become infected with something which is affecting the MB. Remember that its on a Intel MB. A Full wipe of Windows and reinstall should be done.
Well hope that helps. Good luck with that.