I wouldn't keep anything I wanted to keep secret on a networked computer (including one connected to the internet). But while it's sometimes possible to reconstruct deleted data from a hard drive, this does take some fairly sophisticated forensic software and quite a bit of time. For me, and I'd guess it's the same for most people, this isn't really a problem. I'd just as soon avoid the spyware programs that some software can run without your knowledge, and Ad Aware (a freeware program) is pretty good for this. If I had a machine connected to the net at all times, I'd use a firewall such as Zone Alarm.