It's likely to be one of two possibilities:
1. Did you recently upgrade your Mac mini to Lion? FCE is not supported on Lion/Mt. Lion and users have had only modest success running it on Lion/Mt Lion
2. What camera are you using and how is it connected to your Mac? Since you are using Capture Now, I suspect you are using a miniDV (DV or HDV) camcorder and have it connected via FireWire. Do you have ANY other FireWire devices connected at the same time as your camera (for example, an external hard drive)?