To properly open the case, you need to unplug the machine and then press the "start" button. That will discharge that last bit of stored energy (and you'll see the little green ready light on the back turn off). Then and only then can you open it. And be careful touching anything in there if it is a day of static electricity.

I'd use a can of air and blow the dust out rather than vacuum - you could dislodge or remove something you don't want to with the suction.

SRS