If you want to make sure that you're protected from viruses, you should get some antivirus software. This software can offer peace of mind, even for experienced computer users who would be unlikely to open a simple virus.
Antivirus software works in two ways. Firstly, it has a big 'dictionary' of viruses, allowing it to scan files and flag any that are known to be viral. Secondly, the software monitors the system for any suspicious activity. One method involves creating decoy files that are hidden from the user and monitoring them to see if they are altered – if they are, then it was almost certainly a virus.
The two giants of the antivirus industry are McAfee and Norton. They are both commercial software based on yearly subscriptions, and at this point there's little to choose between them – some people swear by one, but just as many will swear by the other. However, both are quite expensive, especially if you buy them as part of the complete security suite.
If you want a free antivirus program, the best around is widely considered to be AVG (the Free Edition). It's a small but effective antivirus application, popular firstly for being free and secondly for using hardly any system resources to run. AVG is capable of running very fast, even on older computers.
For people who don't actually want to download and install antivirus software, but just want to quickly make sure that they don't have a virus, TrendMicro's HouseCall website is a good solution (you can find it at housecall.trendmicro.com) . When you go to the website and click 'Scan Now', it will scan your computer for viruses right then and there, without you needing to install anything – and best of all, it's free. While it is a very good service, however, you shouldn't rely on it completely, as it is a bad idea to use a computer with no installed antivirus protection.