See Borislav's suggestion.

UPDATE. I see it didn't help.

What Windows Updates were installed recently? Could they somehow cause the problem?

>Hi there,
>
>The client PCs have all been rebooted several times. I should have said that this problem started two weeks ago and we haven't managed to resolve it in that time.
>
>It's all very strange.
>
>Matt.
>
>>>Matt.
>>
>>I'm not sure if this would help, but did everyone in the company also re-boot their PCs?