What happens when a computer gets a virus?

It depends what computer virus you have because different virus do different things. For example trojan virus describes a class of computer threats (malware) that appears to perform a desirable function but in fact performs undisclosed malicious functions that allow unauthorized access to the host machine, giving them the ability to save their files on the user's computer or even watch the user's screen and control the computer.

my best advice is defrag and disk clean your computer and then if problems still occur. i recommend getting anti virus software or something to get rid of it!

It depends on what computer and what type of virus