Tigerheart.Hackers wroteShant, hooking the PC up to even a wooden table would work, and no, it doesn't take much. A simple nail would do. We tried it on a dual PSU 850W each. As to where the electricity comes from, it's sort of weird. Sometimes cases that aren't connected to anything, just lying around, give out charges. We're curious ourselves. It has to do with the ground and the surrounding I guess, but what exactly, we're not sure. Not even electricians seem to know!
Actually humidity is a factor in static built up (low humidity = more static built-up), especially in the winter when air is cold and dry outside then you move inside where air is warm and same case for AC cooling in the summer (difference in temperature of atmosphere around the PC case and inside is sort of analogous to that case). As for the source of the charges, they come from everywhere (all materials are made of atoms, so when two objects come in contact, sometimes redistribution of charges takes place leaving two charged objects waiting to either take electrons from a nearby target, especially electronic devices, or dispense their excess charge, especially metallic objects).
As for grounding, I prefer you don't use the metal casing as the static discharge source (you'll get small electric shocks and buzzing from the case, I've actually had the same problem with an OR table, the St. Francis OT-2000, where all the grounding occurs on the metallic casing of the motor and electronic PCBs). Try Nabil's suggestion of using a metallic nail or if you want a better solution, use the metal screws holding down the electronic boards for the grounding process (i.e. ground each board separately).