bjornl :
While not ideal, it is not a big deal so long as the PC was turned off. Just read the ingredients label and see if you sprayed anything corrosive in there. I have not seen any canned air products which contained anything you need to worry about. If you want to be certain, (with the PC turned off) open the PC and use some isopropyl alcohol to clean off all the residue.
Ingredients are propane, butane, Ethane, 3-butadiene,
isobutane, difluoroethane.