ESET's NOD32 has a superb reputation as one of the best anti-virus products around, with excellent intelligent heuristics-based virus detection technology.
Heise Security recently reported that ESET have just launched their ESET Online Scanner, a free Web-based service using the same technology and signatures as their highly-rated ESET NOD32 anti-virus software. Unfortunately this 15MB download is only for Windows computers (lots of versions - Windows 98 / ME / NT 4.0 / 2000 / XP and Windows Vista) and it only works in Internet Explorer 5.0 or later, not Firefox or Opera (see the system requirements and detailed FAQ).
Even if you already have antivirus software, as Heise point out it's best to occasionally try something from a different anti-virus vendor, from time to time, in case your usual software missed something.
This scanner is said to scan for all kinds of malware (viruses, worms, Trojans, phishing and spyware), including inside archive files and email messages, and to clean systems and remove malware, not just detect the malware.
Why are they making it available for free? In the hope that you'll then buy the paid-for version, of course. The free version will scan and clean / disinfect your computer if it's infected, but it won't prevent your PC from being infected in the first place - that's what the full paid-for version is designed to do.
There are decent step by step instructions for installing ESET Online Scanner:
After it's initialised, if you want it to automatically remove "found threats" tick the box before you hit Scan. I did. I don't know whether, if you hadn't ticked "Remove found threats", it would have given you the option at the end of the scan to remove them or not, or if you were stuck with what you originally chose, which is why I ticked it. Similarly if you want it to scan for "potentially unwanted applications", tick that box:
Then hit Scan and it'll whirr away. I found that it scanned all my attached hard drives, including an external hard drive:
Heise said ESET "does its work swiftly", but unfortunately in my experience anyway, it took absolutely ages - 22 hours in total to scan my 120GB internal hard drive and 250GB external hard drive. Yes, that's nearly a whole day. However, to be fair, when I do my standard weekly anti-virus scan with my usual anti-virus software, I have to let it run overnight, and that's just a scan of my 120 GB drive.
As you'll see, it found and tried to clean out some threats on my computer which my existing anti-virus, anti-spyware and anti-adware programs (which I run at least once a week) had missed:
Here's the info from the Details tab, in my case it was mostly the same one or two viruses, e.g. a duplicates in my Recycle bin and backups on my external hard drive etc. Oddly enough I think that WxBug.exe is part of AIM and is technically spyware but doesn't do much so some would call this a false positive. You'll see it couldn't delete some suspect files too:
So overall it seems to do its job, there are mixed views about whether false positives are a good or bad thing. (I know I personally hate too many, myself, but I didn't mind in this case for some reason. I don't use AIM much, we'll see if it stops working now!).
I think it's good to run this sort of online scan once in a while, just to be safe. But I'm probably more paranoid than most anyway and always run a variety of scans at least weekly. Just be aware that online scanners can take a long time to do their thing, possibly much longer than programs you have installed on your computer.
One warning - uninstall it after you finish - Heise point out that "security experts have sometimes found vulnerabilities in ActiveX-based online scanners [such as ESET] which allow arbitrary code to be injected into systems via manipulated web pages... Therefore, ActiveX- based scan software should be uninstalled after the system has been scanned, which in the case of Eset can be done in the control panel via the relevant product entry in the software manager." (And yes it's also called ESET Online Scanner in Control Panel's Add / Remove Programs.)
Other anti-virus software? Some free alternative on-demand malware scanners, which may not even need a Web browser to work, are described and linked to at the end of the Heise post if you want to try them.