While an antivirus software may keep your Windows computer safe, there may be times when you may need to perform an on-demand scan of your computer. When a computer is running there are several files which may be in used and thus locked by the system, making them inaccessible to an antivirus scan.
To get around this, many users prefer to schedule a boot-time scan of the antivirus software to get best results.
On a Windows 7 SP1, Windows Vista SP2, Windows Server 2008 SP2, or Windows Server 2008 R2 SP1 if you run an on-demand scan, not all infected files may get detected by your antivirus software.
This occurs when the antivirus software skips some bytes when it reads the data or file by using a memory-mapped file that is locked.
Microsoft has released a hotfix to correct this problem.
You can apply this hotfix available at KB2698155 only to systems that are experiencing the problem described in this article.
Go here to learn about Microsoft’s recommendations about Windows files and folders you may exclude from Antivirus scans.