Most AV's are set up to scan on file open (reading) and file close (writing). Some provide an option to select one or the other. One that I know of AVG free installs with the default of scan on file open only, but can be changed in the advanced options. Several AV's have a web scanner that scans files as they are being downloaded, but before they are rendered by the browser. This results in redundant scanning as the file is immediately scanned again when the download is finished or the file written to the browser's cache. Why not turn off scanning on file close (writing) when using an AV with a web scanner? Some testing I did with Avira premium revealed that files will be scanned when I open a shared directory on my local network and also when I access optical media. Other than by browser downolad, these are the two main ways that files get onto any machine. If you are thinking about email, most AV's now scan email for nasty attachments. The Avira web scanner even scans archives by default. This is something most on access scanners do not do by default. If I copied an archive from my local network or optical media to my hard drive and then created a file by unpacking the archive and the contents were infected, it would not be detected until an attempt was made to execute the file. At that time it would be detected, assuming it was in the AV signature database. There are other ways to create files like FTP programs and P2P, but ultimately everything gets scanned before it is executed by scan on file open. This is obviously the logic used by AVG in their choice of default setting. Even running a mouse over the file or opening its folder will often be enough to get it scanned. If you don't scan it right away that is all the better because the AV's signatures would be a more recent version. I suppose this gets back to the old chestnut of does one need to scan on both open and close. Its just that web scanning now takes out the largest percentage of files created by most folks and it seems to tilt the balance. Why bother? If your computer is fast the difference is probably not noticeable, although it might give a contemporary notebook computer a few extra minutes of battery life during heavy browsing. Any thoughts?