If you don't mind seeing something that will make you feel a little paranoid, download the Nessus network security scanner.
It is a free tool (used to be open source, but version 3 is closed source but free to download and use because the creators didn't like other people re-branding and selling their work). It installs on Windows, Linux, MacOS, FreeBSD, or Solaris, and it is an easy way to find any known security holes in the machines on your network.
You can download it and install it on a machine in your network. You can install the scanning server portion separate from the client interface, so you can have scanners all over a large network and manage them all from your desktop.
Start it up, and run a scan with mainly the default settings (except turn off the DOS attack scan).
In any well managed network running a scan like this is going to set off a bunch of alarms, and it will probably make your printers print some gibberish, but hackers (smart ones anyway) don't use Nessus like this, they use their own tools, and know how to do it in a less noticeable way.
This scan will generate a nice report telling you all about at the numerous holes you have in your security. Take the ones highlighted in red VERY seriously and patch them ASAP. There is a certain tool out there for taking advantage of all the ones that Nessus says are remote exploitable. ...if not there is a certain search engine for hackers to find exploits. I'm not naming the hacking tools and sites because that is something I feel is best left as an exercise for the reader. :)
For now just run a scan on your network (off hours and with permissions if it is for work) and make sure you know just how open you are. Most people's computers are way more vulnerable than they think, and closing the holes Nessus identifies will make you far less susceptible to viruses and other such annoyances.