UPS

OK, so you're powering two PCs with a single 500 watt UPS and the monitors are connected to another source, so they won't load down your UPS. Effectively, you have 250 watts available to each PC, so if one of them kicks in a HD, CD or just more intensive CPU effort, the power demands will increase. If both PCs are running, they could easily exceed the 500 watt limit, causing the UPS voltage to sag to the point that one of the PCs shuts down. As soon as that happens, the UPS will have extra capacity available to keep the other PC running.

Solution, use a bigger UPS, add another 500 watt UPS or don't use both PCs at the same time.

Grae