A computer virus is a program that is able to multiply on its own without the consent of the user. Virus can encompass all other types of malicious programs such as malware, spyware, etc even though these programs do not have the ability to multiply. Virus from one computer can be transferred to other computer via internet, USB pen-drives, portable hard-disk drives, CDs and DVDs. Viruses also have the ability to infect the files that are present in the system. These are mostly .exe (executable) files.
First and foremost thing you can do to protect your system from virus attacks is to scan your system thoroughly by means of antivirus. It is advisable to update the virus definitions and disconnect the internet connection to your computer before you perform a system scan. Once you are done with the scan you can reboot the system into the Safe Mode and again perform a virus scan.
If you are not able to remove virus from your computer in spite of performing antivirus scans several times on your computer, you can try to find out which computer virus strain has infected your system. You may do a Google Search for the virus name with its removal methods. You will find hundreds of virus removal methods online.
Some virus removal software may prompt you to download a new program to remove the virus from your computer. In such a case, you must see that the source website from where you are going to download is a legitimate one. After downloading the virus removal program, you should scan it to check if it is a spyware or virus masquerading as a legitimate program. There are umpteen number of fake computer security programs available online for free. Such virus removal software are not fake but virus and spyware programs that are aimed at infecting computers of those people who are desperately in need of an virus or spyware removal program. Ideally you may also try installing antispyware and firewall program to make the security of your computer stronger.