There's no other major item most of us own that is as confusing, unpredictable and unreliable as our personal computers. Everybody has questions about them, and we aim to help. Here are a few questions about computers I've received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability. This week my mailbox contained a question about security software for Macs. If you have a question, send it to me at mossberg@wsj.com, and I may select it to be answered here in Mossberg's Mailbox. Q: There's been a lot of press...