Kevin,
All PC Power supplies contain at least one large electrolytic cap. with several 100 Volts of working voltage and at least several 100's uF capacitance. That will do the job. It doesn't matter if you have to put wires of a few cm's length on the cap or on the board to be able to solder the short connections.
Just make sure you connect the polarities correctly.
Since your "new" temporary cap hasn't been used for a while and it may not be of high quality, it may not like the voltage applied suddenly.
Common countermeasures, like a series resistor in a temporary fuse-like role, wouldn't be completely straightforward because of the unknown power supply / load situation. OTOH, I don't think the 100 V suffers from large load or even overload so I'd put in a low-wattage resistor of say 100 Ohm, 0.25 Watt in series with the new, temporary C. The resistor will burn up if the new C has a real problem. Even with 100 Ohm in series, I think your 100 V AC will become significantly lower if C4439 is the problem.
After a while, it may be safer to remove (short) the temporary resistor.
Raymond
