Desktops draw anywhere from 50 to 200 watts, IIRC.
So make a PC power supply that has 3 external connections. One for the wall plug, of course. And two optional ones, one for a 12v solar panel and one for a battery. Then sell it with a 50w or 100w solar panel -- most importantly, the panel should not generate more power than the PC uses.
Because of that, during the bright part of the day, the panel will be providing most, or just barely all, of the power for the PC. The wall plug will provide the rest. At night, the wall plug would provide all the power. It's a grid-tie but it doesn't feed power back to the grid, it just reduces demand on it. The 100w panel takes 100w off the grid load during the peak demand times.
This url might also be useful
http://www.seyboldreports.com/SRDP/0dp9/D0901010.HTM