No, Defender is only for anti-spyware - it does nothing to protect you from viruses, malware, worms, trojans, bots, and all the other nasty stuff that can infect your system. It is definitely not sufficient on its own anywhere - even at home.
Personally, I do not like McAfee (but nowhere near as much as I dislike Norton). I've seen it miss too many things and it causes problems with quite a few programs (though apparently it hasn't for you until, perhaps, now). Remember, while I'm thinking it might be the culprit, we have yet to prove that. It may not be involved in the problem at all - but the chance it is is high enough to want to rule it out as one of the first steps.
To be perfectly honest, if I were you, I would uninstall McAfee using the removal tool (
Download McAfee Consumer Product Removal Tool 3.5.109.1 Free - Will remove all 2005, 2006, and 2007 versions of McAfee consumer products - Softpedia - if you have something more current, go to the McAfee site for most current tool they have available) and install Microsoft Security Essentials
http://www.microsoft.com/security_essentials/ for day-to-day protection from malware, viruses, spyware (it deactivates Defender automatically because it includes that and running both at the same time would cause conflicts) and everything else. Set it up to automatically update (Windows Update will then do that as it now does for Defender) and I'd set it to automatically do a quick scan every few days when the computer is on but you aren't using it very much and a full scan every week or two (I actually do a quick scan daily and a full scan weekly - but it's up to you). Then download Malwarebytes (the free version) at
Malwarebytes and update it and do a full scan every few weeks to catch anything that MSE might have missed. Don't schedule this - just make it a habit to do it (or put it as a recurring event in your calendar). That's how I'm protected and what many here recommend - and I've never been successfully infected and have had no problems or conflicts with either of them.
Do NOT install and run more than one AV program at the same time or you may cause the system to freeze, crash, stall, conflict or cause all sorts of issues. Malwarebytes is an exception to this as it anti-malware as opposed to AV (and because it is an exception - I know of some others but those will give you adequate protection). Of course, be sure to re-activate Windows Firewall if McAfee shut it down as you do want firewall protection and Windows Firewall is a great product and nothing on the market provides better protection (though some products may provide more configuration options).
These suggested replacements are all totally free and, IMHO, are better than using McAfee - so you aren't losing any money by switching over (though it may feel a bit like that if you paid for a McAfee subscription and it still has time remaining - but you wouldn't be spending any more money by switching and it would improve your protection and it may resolve your connection problems).
And you don't seem very shy to me from your posts. Most people (not everyone - but I think most) would be more than happy to help you (and take a break from whatever they're doing) if you ask politely and the right way (you know what I mean). Besides, you do want to get your system working and that's one way (if the manager or their tech guru can't help) of possibly getting it done on your own. I can't help you because I'm sure it has to do with how their system is configured compared to how your system is configured and while you can tell me about your configuration, without knowing theirs, I wouldn't know what to change. Good gosh, you run your own business - how in heaven's name can you do that and call yourself shy? LOL!
I hope this helps.
Good luck!