Having a antivirus which scans your pc and find no virus does not mean yo pc is clean. Antivirus software depend on signatures to catch viruses, if a virus is latest and there is no signature for it, regardless of how good yo antivirus is, it will not be able to catch it and the virus will be roaming yo pc freely!
Imagine a policeman trying to catch a thief, the policeman will need details of the suspect( read signature fo antivirus) which might be his id number, photo, and other infor. When they get him, they have to positively identify him, get him and compare the infor they have. This is same like antivirus looking at a suspected file(virus), run a formular and compare the results with signature datatbase it have. If it matches any signature, then a virus has been found, if it does not then no alarm.
Resources permiting, its always advisable to run two antiviruses together on a pc.