|First, did you remove the heat sink from the CPU (or any other for that matter). This is important because if you did remove a heat sink, then you will need to remove all of the thermal compound from both surfaces and replace it with the proper amount of new thermal compound.|
(Read this: http://www.arcticsilver.com/methods... )
Second, How did you clean it? You should have used a can of compressed air sold at computer and office supply stores, you should NEVER use an ordinary Vacuum cleaner since the plastic hose can carry enough static charge to damage certain internal components. This is also why you did not need to remove any connectors.
Third, Are you absolutely sure that all connectors are connected to the right places? While most connectors look different and can only go one way, there are a few to the case front panel that can be attached in many incorrect configurations and there are a couple of possible connectors that look alike and at least one pair that can be switched. Are you sure that both of the motherboard connectors (20 or 24pin plus 4 or 8 pin connectors) are connected (the smaller one near the CPU typically).
There are some other possibilities depending on how much you disassembled the machine like missing motherboard posts and even missing screws (some become the grounds), Pinched wires, and even failure to ground yourself (static charge) before touching any internal components since a static discharge can damage certain components.
Go over everything again and report back with as much details as you can. Be as honest as you can with your assessment since it will help narrow things down faster and easier.
You have to be a little bit crazy to keep you from going insane.