antivirus software can protect computers against known viruses. some antivirus software scan files and compare a hash code for each file with its database of hash codes (or "signature"). if the code matches, it has likely found a virus. this way of doing things has created some problems. it will only protect against viruses whose hash code is known. the companies that wrote the antivirus need to keep the virus signatures up-to-date and need to give this information to the pc to be protected.