It could be a ton of different things. Could be drivers, software, infection, windows corruption, bad network card, and the list goes on and on.
So what I do is I go down a check list and start marking things off to narrow the search. Otherwise we would be wasting time trying to fix something that isn't broken. Once I have the list narrowed then I try to find the common factor. At that point I normally have an idea of where the problem is and what to do to fix it
So on that list is taking the current network you are on out of the loop (Work network). This way this test will point me in the right direction and we can avoid wasting time on the system if it is the network, or wasting time if it isnt
Then as you guessed the next thing is trying a network cable instead of wireless (After trying the wireless at different locations), this test tells me if it might be the wireless card or drivers and so on. So as we start going down the list you will see where the problem is one step at a time :-D
Personally I have seen things like this happen and each one has been a different reason. Lots of time it has been the antivirus software with a built in firewall that was the cause. Software firewalls are pointless, a router is a hardware based firewall, so unless you are wide open to the net you dont need a software firewall normally, and what happens is the user will click to block the wrong thing and now Windows doesn't handle the network correctly. Also seen it to where updated drivers might fix it as they have bug fixes and better support. Seen it be the router, and see tons of other things.
So bare with me as I talk your ear off to try and find the problem lol
Shane