It's not rusting, that's the color of the electrolytic fluid leaking out of the capacitor.I opened up the case again and noticed that the large capacitor on the power board is rusting. Something is leaking out of that thing.
The fluid is the problem - it's a case of industrial espionage gone wrong - they stole a bad electrolytic fluid formula Badcaps.net is a fascinating site if you are into electronics, geeky in nature or just enjoy a good mystery. Dell got hit particularly bad with their Optiplex GX270's - we had to eventually replace thousands of them and it go so bad that for a while we stocked internally a steady supply of motherboards to swap out as the machines turned up. We eventually started to proactively look for the bad ones - the capacitors, at least for the Dells, all had an X on the end of them so they were easy to spot. If there as any other letter they were fine. It was a complete nightmare - but as others pointed out, it affected the entire electronics industry and not just one or two manufacturers.