The program can check files before they ' re opened , and most av programs are configured to check executable files by default 反病毒程序能在这些文件被打开前进行检查，并且大多数反病毒程序默认配置为检查可执行文件。
Antivirus programs can scan files already on a computer ' s hard disk and files ( such as email messages and downloads ) as they come in over the network 反病毒程序可以扫描计算机硬盘上已有的文件和通过网络传来的文件（如电子邮件信息和下载） 。
Finally , host - based av programs are under the control of the individual user . users may turn off the av program or change its settings , or open files that have been quarantined 最后，基于主机的反病毒程序处于个人用户控制下。用户可以关闭反病毒程序或改变其设置，或者打开已被隔离的文件。
Mail server - based av programs that scan incoming and outgoing e - mail messages and attachments for viruses and intercept them before they reach users ' mailboxes or are sent out over the network 基于邮件服务器的反病毒程序对进入和外出邮件信息和附件进行病毒扫描，在有毒邮件到达用户邮箱或发送到网络前中断。
If a company relies on host - based antivirus to protect the network , they may overlook laptops that employees bring in to connect to the network or home computers from which employees connect remotely 如果公司依赖基于主机的反病毒程序来保护网络，他们可能没注意到员工带来的接入网络的膝上型电脑或者员工远程连接的家用电脑。
Host - based av programs usually do a good job of detecting viruses in email and are essential to protect against viruses introduced locally ( for example , via a floppy disk , usb key or cd ) , but don ' t protect as well against viruses that come in via web pages or im 基于主机的反病毒程序通常在检测电子邮件病毒和防止本地引入的病毒（例如，通过软盘， u盘或cd ）方面做得很好，但在对通过网页或im传播的病毒表现不够。
