Good question! Just FYI, you don’t need a “program” if all you want to do is a full screen shot. Just hit the “Prnt Scrn” button on you keyboard, which will copy your screen to the clipboard. Then you can paste it into GIMP, or Microsoft Paint to save it as a jpg file.
However, if you want to “upgrade” your screenshots a bit, you will want a program designed specifically for screenshots. I searched for quite a while before I found one that I like. It’s called Shotty, and you can find it here. The main reason I prefer it above most screenshot software, is that it can preserve the rounded corners/transparency of windows on Windows 7/Vista. It also allows you to set your own key combination for capturing the screenshot, and you can also configure it so save the screenshots automatically to a folder!
Hope that answers your question!
P.S. If you ever suspect something that you’re about to download from the internet, go to http://www.virustotal.com and scan it. It will scan the file/link with about 40 different antivirus software, so if it doesn’t find anything, you’re probably ok! The best way to avoid getting virus’s is to pay attention to the website you’re downloading it from. Take Skype for example. If you want to download skype, ALWAYS go to http://www.skype.com to get it. Don’t search google for it, then click on the first link you see, which could be www.downloads123.skype.com and is more likely than not infected with some sort of spyware, or worse. If in doubt, scan it with virustotal, or ask someone who knows more about it.