I bought this

Data Flash: Airduster

Is this safe to use to blow dust from my heatsink and motherboard, hardrive and fans etc?

yes it is perfectly safe to use, in fact it is the only really safe way to clean dust out. just dont turn it upside down or spray it on a single area for too long, as the very cold air can damage coponents

Yea i tried it on my hand to see if any liquid came out , got a bit cold lol

Two more questions ^_^

What would happend if i tipped it upside down?

Will a 400ml last me long?

should just do the same even upside down and how long it lasts depends on how dirty you're computer is inside and remember not to hold the nozzle to close to the parts that you clean if there is not a lot of dust it should last you a while as you only really need to clean the insides once a year unless in a v.dusty environment

if it is turned upside down, the compressed air inside the can is liquid,and the liquid will come out. it can cause instant frostbite on contact, so i wouldnt reccomend spraying yourself with it. a 400ml can should last a while depending on how much you use