Even if you have a virus scanner on your computer, having an extra one is often a good idea if you're installing something that you downloaded from the internet. Two minds, as they say, are better than one. Although 43 are even better, and that's what VirusTotal gives you. Upload a file and, within half a minute or so, it'll be scanned by no less than 43 different programs and the results instantly displayed.
I make a point of using VirusTotal (www.virustotal.com) to check every file that I recommend on this site, whenever possible. One problem, though, is that the site has always refused to check files larger than 20 MB. In the past couple of weeks, following a redesign, the site now accepts uploads of 32 MB. So if you're unsure about the precise provenance of a file or document that you've been sent, now is a good time to get into the habit of using VirusTotal before opening or running it on your computer.
I also wrote recently about a great free online utility which converts PDF files to Word format. Again, the site has an upload limit of 10 MB. However, if you need to convert a file that's larger, you can download a free desktop version of the program that runs under Windows. You can get it from http://www.free-pdftoword.com/ and, needless to say, I've checked it with VirusTotal. My computer has spent much of today converting over 1,000 MB of PDF files, some of which are 150 MB, and the program is making a great job of it. It's only a 7 MB download, too.