Try a Fluoro ballast or a flyback xformer from a TV/CRT. make to electrodes of your desired length. Then play with the spacing until an arc can jump the gap.
You are going to want to put a cover over it when you get the spacing correct. I dunno about putting anywhere near your computer case though. Maybe make it resident on a shelf several meters away from your computer equipment. You're asking for EMI/Static problems if you put it on top of your comp case.
ALTER THE SPACING WITH THE POWER OFF! I TAKE NO RESPONSIBILITY FOR ANY ELECTROCUTIONS! YOU WILL BE WORKING WITH SEVERAL kV!! I WARNED YOU!
BDOG