Yes, aadipa is right. But many AVs use this method, and this is called "Integrity Checking". This is also used by AVG, Avast etc.
AVs scan entire disk and recod the file and disk information, and they create signatures of them, when they are scanned for 1st time.
Then, next time, they simply compare the file with their signature, and if they dont match, the AV engine will scan the file for suspected viruses. If the files and their signatures match, then it will not scan the file for viruses.