If you're surfing the web and, all of a sudden from out of nowhere, something other than anti-virus software that you yourself have installed greets you with a notice that your computer is infected and telling you to download a program to remove the virus, please don't do it. It's a scam. Clicking on the so-called remover is what puts the virus onto your system. It will then take a non-trivial payment to someone to have it removed. The same site that placed the virus on the computer will offer you a program to remove it. Fool me once, shame on you. Fool me twice...uhm, shame on you again. You're not going to get me a third time, but should anyone chance a second time? Maybe better to give the money to a trustworthy professional.
However, if you are using a sandbox as we discussed last time, all you have to do is empty the sandbox as you thumb your nose at whoever was trying to get at you.
Whenever your computer acts in a way that makes you uncomfortable, or every once in a while just for the fun of it, it's a good idea to run an online virus checker, such at Trend Micro's Housecall. (There are others. Google is your friend.) The advantage of these programs is that they do not reside you your computer, so it's harder for a virus to evade them.
Every once in a while, you may come across an online utility that looks too good to pass up. Some of them are. One of my criteria for whether such a program is likely to be safe is whether the site offering it has another source of income. If it does, then the utility is likely a form of advertising. If it does not, be afraid. There are some precautions you can take. You can type the name of the program into Google along with the word "virus" to see what pops up. Another option is an online program checker, such as Jotti's malware scan. (Again, there are others and Google is your friend.) These free services allow you to upload the program, which they then run against a suite of virus checkers.