There's no reason to be paranoid. If you don't go to porn sites, don't download illegal torrents, don't download from crack sites, don't download and open email attachments with double extensions (e.g. .jpg.exe) and don't transfer data from other PCs to yours (e.g. using a USB stick), then the chance of getting a computer virus is lower than getting hit by a meteorite.
By average my anti-virus detects a virus on my USB stick once in 2 months (gotten from another PC) and maybe once in 6 months while being online (usually while looking for a rare piece of data / software / torrent / game). So I would get more often infected by offline content.