The time it takes to start the PC, known as the boot time, is also increased by the operation of security software, sometimes markedly.
Look for security software that has minimal effect on PC performance.
Security software vendors often bundle related software with anti-virus. These include firewalls, parental control software and online backup systems. While potentially increasing security for the user, the added bloat can also slow the PC further.
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Anti-virus software vendors have a sales model built around annual subscription. Typically priced from £25-50 per year, a subscription entitles you to download updated virus definitions for the latest known Windows malware.
When the subscription period expires, anti-virus companies advise you to renew your subscription in order that you can receive updates to their ever-changing database of malware.
Sometimes AV software is licensed per single computer, or for a fixed number of PCs - eg, a three-seat license.
Since many homes have more than one Windows PC, if you want to install anti-virus software on them all, look out for 'household' licenses which allow use on as many PCs as are in use by a family.
Mac OS X and Linux are desktop operating systems that do not attract anywhere near the level of malware as Windows. Security software is available for these platforms, usually to help screen out Windows threats and prevent them being passed on to Windows-using colleagues.
At present, there are no viruses in the wild that affect Macs, for instance, although there are some isolated reports of Trojans. These require the explicit consent of the user to be installed, by typing in an administrator password.
There are reports of malware designed for the Google Android mobile platform, and security software vendors are now releasing anti-virus software packages to help users fight the dangers. Google, meanwhile, responded to the last raft of Android malware by removing apps from its Market that were known to be hazardous.
No malware exists on the official Apple iPhone platform, although handsets that have been unlocked to run unapproved apps ('jailbreak')are at risk.
RIM BlackBerry OS likewise has no known malware in the field.
Windows Mobile did attract various forms of malware, and this can still pose a threat to users of Windows Mobile handsets; the revised Windows Phone 7 OS seems to have avoided the attention of virus writers so far.