Thanks Rman I'll try and give a little more detail.
Firstly this is a problem that our computers only suffer with when using our network. Friends and public wifi spots are fine and uninfected.
Although the problem was present on Pcs and Macs on the network, it is now only worrying me on the macs as all pcs on the network have had a fresh installation + security update and are awaiting the problem to be resolved on our macs before being reintroduced to network.
The issue on the macs is now this - when following a link on google, instead of taking me to the requested site I see the address bar at the top of firefox scan through various sites and eventually end of at a random page.
This random page usually includes my original google search term but bears no relevance to requested page - ie band's myspace page.
I have done some online research into this and have found various pages: -
The first seems to come to the conclusion that the problem we are having is a trojan named - OSX.RSPlug.A.
How to Remove the OSX.RSPlug.A Trojan Horse from your Mac | eHow.com
However the solution detailed on the page does not solve the problem.
-Virus Barrier has found no problems.
-There is no file entitled 'plugin.settings' in the Internet Plug-Ins folder
-I have opened Terminal and followed the insturctions from the page: -
Type in "sudo crontab -l" (the letter L, and minus the quotes), hit Return, and enter your administrator password when asked. If it returns with anything other than "crontab: no crontab for root", you are most likely infected.
And my mac does return with "crontab: no crontab for root"
This makes it seem as if I am not suffering from the problem but the redirects on google, ONLY WHEN USING OUR NETWORK, still continue?
The second page I have found on this subject/problem is this: -
Macworld | First Look: Trojan Horse warning: What you need to know
This page again talks of the same solution, by deleting plugin.settings file (which is not present on my macbook) and then checking by doing the same terminal check which my macbook passed.
However this page does advise you to check in System Preferences/Network/Advanced and then viewing the DNS tab. Here, the pages says you should have no grey DNS servers listed. I have three?!
Is there a way to check where each one of them is being received by?
Why would I have three?
How do I remove any of them as they all have greyed out minus buttons?
Many thanks to anyone that can help with this.