VPN connections not connecting

Some days at work I really wish I was a cat which only has to worry about when they are next going to be cuddled and have to make the decision as to whether they should eat some more or sleep some more.
Today I had to go round a colleagues house to sort their vpn out

– as it turned out, their fault was easily fixed – it just needed the correct ip address entering into the vpn settings (I had mistyped it when configuring their desktop.) All went suprisingly well and I was a bit disappointed as I hoped that by solving this desktop I would be able to solve another desktop problem, but alas no – this one was an easy fix.
Then at 12.30 I got a message saying that the vpn had stopped working and that when they vpn’ed in, they couldn’t access anything on the internet. Looking at the desktop, the pc was making the vpn connection to the office and getting the correct wins and dns settings, but no traffic was being returned back from the office to the desktop – traffic was being sent out, but nothing received.
After 4 hours of trial and error I’ve narrowed it down to the fact that when this pc connects on a wired connection to the router the traffic doesn’t get past, but when connected on a wireless connection traffic is passed ok and everything is fine. However, if I connect *my* laptop to the same wired connection it works fine.
Having said that, for about 4 hours I was unable to get the new machines to connect at all but mine could – then all of a sudden the wireless connections started behaving. The weird thing is that even when the desktops were connected to the lan, they would not appear in the active connections of the management console.
I’ve told them for the time being to try the wireless connection, but first attempt the wired connection to see if that suddenly starts behaving too. I’ve no idea why the laptop I looked at on Tuesday doesn’t work – it has the same symptoms but no wireless card so the solution can’t be to use the wireless connection.
A very strange and frustrating day. I spent 4 hours troubleshooting for something to “just start working”, the work mate got no work done today as this was on their (new) only machine and I’ve still no idea what the problem was.