Every computer virus ever written takes up disk space, and many of them are able to remain in the computer's memory taking control over some computer functions. In addition, some viruses are poorly written and may cause the computer to halt, or damage files.
Many viruses make the computer's memory unstable, or cause programs to run improperly.
Then there are viruses created in recent years that have been deliberately designed to destroy data on the disk. Sometimes viruses will even be on a timer and won't start replicating until a certain time or date, like the April Fool's bug, the millennium bug, etc. However, legitimate software can also cause these effects. And while some viruses are very obvious, displaying messages, or even playing musical tunes, many give no sign of their presence. So it's important not to assume your computer is infected, just because strange things happen.