Computer shutting off durring Boot

I decided to give my computer a good dusting because i haven't dusted it in awhile. This was the first time however, that i dusted my heatsink and Power Supply. When i put them all back together i found out that my computer beeps twice real fast a few seconds after boot up and promptly shuts down. After looking around i found out that the problem might be the grease or paste (i forget the name of the stuff) that you put between the heatsink and the CPU. I seriously doubt that i harmed the Power Supply but i did take it apart and check all the wiring, and i checked for loose wires all over the motherboard.
So my question really is could adding new grease and cleaning the old off possibly fix this problem? And if so what compound should I get and how should i put it on?
I have a Asus P5LD2-VM which is a socket 775 and and dual core 2.8GHz Pentium Processor

 

Check very very carefully that you have the processor in the correct way round and that no pins are bent.

 

I only took the Heatsink off and not the Processor.
Another thing, does the two fast beeps before it shuts off mean that the Processor is overheating?

 

No but you might have disturbed it in its seating.

It's obviously something you have disturbed so you must meticulously check everything, including things you did not specifically remove but may have disturbed.
Did you reconnect the internal fan in the psu and is it still working?
Is the processor fan working?
Have you reconnected all the psu connections to the motherboard and graphics cards?
Are the memory modules still firmly in place or have you blown some dust into the contacts?

A poorly assembled heatsink will not halt the system, just allow it to overheat eventually.

 

Sweet it works now ^^
Not sure where the exact problem was but i had unconnected only what i messed with and reconnected it before with no results. This time i did what you said and unconnected pretty much everything and gave it a good solid reconnection (IDE, all motherboard wires, PCI cards, processor, ect.) I also put on new thermal compound, though that probably wasn't the answer.
So i guess something somewhere was connected badly, and i have to admit i feel slightly stupid for not finding it.

Thanks for all your help studiot ^^

 

There you go

:major:major