It is important for every Windows machine to have anti-virus software installed, continually updated, and continually scanning.
There are a lot of good anti-virus programs and protection suites on the market. Some are good on ease of use, some are good on scanning speed, some are good for managing multiple machines on a network and some provide good basic protection for free. Read the reviews below and choose the product that has the features that you need.
Comparison of antivirus programs
www.av-comparatives.org/
PCWorld review of AV programs
www.pcmag.com/reviews/antivirus
PCWorld Best Security Suites, 2012
www.pcworld.com/article/248479/best_security_suites_pc_bodyguards.html
PCMagazine Top AV Prorgams for 2013
www.pcmag.com/article2/0,2817,2372364,00.asp
PCWorld review of free AV programs
www.pcworld.com/reviews/collection/5928/2011_free_av.html
Download list of free security software for Windows from C|Net Download.com
You should install only one 24/7 monitoring antivirus software, having two active AV monitors invites conflicts between the two.
There are some online scans that can be done if you suspect your machine has a virus www.eset.com/online-scanner
www.bitdefender.com/scanner/online/free.html
For on-demand scanning (as opposed to 24/7 monitoring) I recommend Malwarebytes which has both free and paid versions.
SuperAntiSpyware is also popular.
You should also have a firewall turned on, the Windows Firewall is a minimum, but the firewalls from ZoneAlarm and Comodo will both monitor outgoing as well as incoming connections.
Having an anti-virus scanner installed is not enough, you need to learn how to browse safely and configure your machine for security. Engaging in risky behavior can allow malware into your machine around your security software. The first defense against malware is the powerful computing device between your ears 🙂