Once you have installed an antivirus or internet security program you shouldn’t have to scan it again. Or at least thats the theory anyway.
If you have protected your computer with a decent security system then a scan should be run as part of the installation process. After that, the program should be shielding you from malware by automatically detecting and blocking malware at the point it reaches your computer, be it from an external device, web page or anywhere else.
Of course thats all well and good in theory but real life doesn’t work that way. The bad guys are always coming up with new viruses, worms, trojans, etc and, as soon as they see that they are being blocked by security software, they will come up with something else with the intention of circumventing that block. Many antivirus scanners can be caught out by this. Sure, the virus definitions database will get updated, often quite quickly too, but there is always a chance that you could be infected by a new malware variant before your security software has been equipped to deal with it (zero days).
For that reason you shouldn’t abandon scanning your system altogether. Opinion on how often you should conduct a scan, be it scheduled or on demand, is quite divided and whats right for you may well depend on how you use your machine, including the devices you attach to it and the places you visit on the web.
I would suggest that scanning your computer every week or two should be adequate for most users. And I would also suggest that you set up scheduled scans just in case you forget to run one at any time. It may also be advisable to choose an antivirus scanner that incorporates a level of cloud protection as these programs tend to be able to deal with 0-days much quicker than other programs which rely solely upon the increasingly outdated database model of protection.
How often do you scan your computer?