The causes don't matter so much as your ability to deal with them. The gretaest cause is weather. Not much you can do about that.
Get the PC off the carpet! The floor it the worst place to put a PC because it also collects dust in that location and the charge from the carpet can migrate to the dust in the PC. That dust by itself will attract static electrical charges out of the air also.
I'm guessing it has a lot of dust in it. So take it apart and get all the dust and dander out with compressed air, vacuum, etc. Be careful! If you are standing on the carpet a discharge from your finger to a circuit in the PC can destroy components on the mobo and/or disk drives. Once you get the covers off and everything apparent cleaned out, remove all the components (expansion cards, disk drives, psu, mobo, everything) and get the dust that is under then out also. Don't forget to take both side covers off if it is a tower or minitower. Then clean out all the nooks and crannies and connectors as well. You probably need to take the cover off of the PSU and clean it out also.
Once you get it put back togethert, put it back on a desk, side table, whatever - but NOT on the floor.