If you go into a school with existing Windows computers, chances are they are overrun with viruses. This can make the systems mostly unusable. Whether you're cleaning up what's there or doing a fresh install, anti-virus is a must.
One problem that comes up is this: anything that costs ongoing money is not going to get updated. I have seen tons of machines with expired Norton or McAfee subscriptions that were years out of date and overrun with viruses. Free systems that require periodic re-installation like AVG aren't much better.
For Windows XP/Vista/7 systems, I've had the best luck with Microsoft Security Essentials. Both installation and updates are free and automatic when online. It's also unobtrusive and requires no user interaction.
You can also download the virus definitions manually and bring them around on USB if you want to do updates in areas without internet. I have kept several remote schools virus-free for years by doing just yearly manual updates.
Windows 8 and 10 actually include this software already, but they've changed the name to "Windows Defender". It's the same program, and uses the same updates. However it is only enabled if you uninstall other anti-virus software. I've seen systems that sadly would have been made safe by the default Windows Defender installation if only someone had uninstalled the expired fee based anti-virus!
It's also just a good idea to remove other anti-virus from any system, as having multiple anti-virus programs installed will slow systems way down as well.
These are just my thoughts from a few years in rural South African schools. Chime in if you have other ideas.
note: to confuse people Microsoft re-used the name Windows Defender... if you are on XP/Vista/7, Windows Defender will not protect you from viruses. You want Security Essentials. If you are on 8 or 10, you will use Defender, which is already installed.