1. Computer Virus can slow down your computer performance and become inefficient or run slowly.
2. It tends to fill up the computer with useless date.
3. Virus can corrupt the system files.
4. Virus can also wipe out the Boot Sector creating problems when you boot into the windows, which contain system information.
5. It can make the changes in the data of the programs or files and cause erratic results.
6. It might steal important information from your computer and send the same to some other person.