a) A paid subscription to anti-virus software has expired and is no longer being updated (therefore your computer is not protected from new threats)
b) Since anti-virus software has been installed, the computers performance has slowed right down
c) There is no anti-virus software installed at all (people don't want to pay out for a subscription or don't know what free anti-virus to download)
A solution to the above which I highly recommend* is Microsoft Security Essentials.
It is completly free (for home users only) and can be installed on Windows operating systems Windows XP Server Pack 3, Vista, and Windows 7. (Windows 8 has a built-in AV component)
And as its Microsoft software, it ties in nicely with your Windows operating system and Windows Firewall and I have found it does not slow your system down (compared to other Anti-Virus software I have used)
*Please note that I am recommending this if you are looking for free Anti-Virus software. It provides the basic protection needed, but Anti-Virus software you pay for will almost always offer you more protection. PC Pro Magazine did a comparison and BitDefender Internet Security 2013 was their winner (costs £15 and avaliable from www.amazon.co.uk)
Firstly, you will need to uninstall your existing anti-virus software
First of all you will need to uninstall any previous anti-virus software. To do this, On XP, Start > Control Panel > Add/Remove Programs. Find the program in the list (i.e Mcafee or AVG) and click uninstall.
In Vista/Windows 7, click the start icon > Control Panel > Programs Uninstall a program. Select the program from the list and click uninstall,
To download, open internet explorer and browse to http://windows.microsoft.com/mse
Click the ‘Download’ link
At the prompt. click ‘Run’
Click through the install wizard.
At the 'optimise security screen', ensure ‘If no firewall is turned on, turn on Windows
Once installed, the wizard will ask if you want it to scan your computer for any threats. It is recommended you select this box.
Microsoft Security Essentials will check for Anti-virus updates daily and will update automatically.
To check it is up-to-date, open Microsoft Security Essentials, either in the Start Menu or there may be an icon on your taskbar. If the taskbar icon is green, it is fully protected.
When you open the Microsoft Security Essentials Console, you will see the following options (screenshot above). A brief summary of each is as follows:
Home tab: A green tick is a signal that your computer is protected and has up-to-date virus and spyware definitions (basically meaning your system knows about all the latest viruses and spyware out there that Microsoft are aware of). Here you can also start a quick, full or custom scan of your computer.
Update tab: If your virus and spyware definitions are not up-to-date (Red cross on
home screen) or you manually want to check for any new updates, click ‘Update’
History tab: Check history of jobs and detected items
Settings tab: Schedule a scan. The default is weekly, Sunday 02:00.
You can exclude files from the virus scan and set what the system should do if a virus or spyware is detected. (The default is ‘recommended action’ which allows Microsoft Security Essentials to automatically decide whether to quarantine or delete based on what type of detection it is)