With an up-to-date operating system and browser, and updated anti-virus software, catching a digital nasty is actually fairly rare.
But they are still a threat to your computer, and can also spread quickly via social media networks.
At their worst, viruses can corrupt your data, or share it with criminals. If a virus is lurking on your computer, knowing how to spot the symptoms is vital.
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Spotting the symptoms of a virus
If you have a virus, it won't flag itself up with a skull and crossbones symbol, or line of binary numbers scrolling vertically down your screen.
The real symptoms of a virus are far more subtle, and often mundane, but there are still tell-tale signs to watch out for.
A slow-running system
Viruses can clog up your computer, either using up its memory to perform its own tasks, or causing conflicts with other programs that in turn gum up the system.
If your PC performs like it's wading through treacle, a virus could be to blame.
Frequent program or system crashes
If you computer suddenly starts shutting down or crashing without warning, or if individual programs start to act up, it's time to run a virus check. The same advice applies if individual programs start to act strangely.
The Windows Reliability Monitor can help spot issues. Go to the Start button and click Control Panel, and then, under System and Security, click Review your computer's status. Click Maintenance, then, under Check for solutions to problem reports hit View reliability history.
Random Windows messages
Infected PCs could generate multiple warnings from within the operating system and it might also infect programs that you haven't selected.
Problems accessing files and folders
As a virus spreads, it can corrupt files and folders, which makes it difficult to navigate and access files. Sometimes even opening a folder can cause your system to hang or crash.
Limiting the damage of a virus
Once you have cleared the malware from your PC using your security software, we recommend changing your email password, in case this has been compromised. Don't do this until you have run a full virus scan, as an infected PC may have key-logging malware to lift your password details.
If you realise you have a virus that is trying to spread or send spam via your social network friends and email contacts, then as well as getting rid of the virus from your own machine, it makes sense to warn them. Apologise for the issue and warn them against clicking on links within your last emails.
Watch out for...
Technical support scams
NEVER give anyone who cold calls you claiming you have a virus and saying they can fix it permission to access your computer. They are lying, especially if they claim to be connected with Microsoft.
Microsoft will never contact its customers directly, and it doesn't have any way of knowing if your PC is infected or running slowly. Error report messages are sent anonymously from PCs.
Virus threat pop-ups
Rogue pop-up messages known as 'scareware' may appear when you're browsing the web. These may not look like the messaging from your own antivirus software.
That's because these messages are imitating a virus warning in the shape of a pop-up saying your machine is infected. The safest option is to close down your browser without clicking on the pop-up itself. Any interaction with the pop-up, including the X-symbol for closing it, could potentially download malware to your machine.