So second level support calls in: "Listen, one customer installed our application, but he keeps on insisting that the user interface is all empty". "Yeah right", you think, thanking god that you don't have to answer hotline calls.
A quick Google search brings up a well-known suspect: McAfee AntiVirus. According to this Microsoft knowledge base article, McAfee's buffer overflow protection feature can screw WinForms applications - and that's exactly what had happened at the customer's site. Luckily there is a patch available.
BTW, I stopped using McAfee privately when they changed their virus definition subscription model - I hate to pay for a program that literally expires after 12 months.