Re: What's the real danger ?
No connection to the web? And the client PCs that access it? Do they have access to the web?
Also are you in an industry that has any form of compliance or ISO 9001? If so then it will cause audit problems.
Do your customers require you to be compliant with any form of compliance or certification or are they certified? If so, you may have problems delivering to them, if they find out during a compliance request that you are using unsupported software.
Is there any personally identifiable data on the machine (emails, email addresses, CRM database, personnel records etc.), if so you might have a DPA violation on your hands - I'm not up on UK data protection, in Germany it would cause headaches.
If you are only selling to consumers and the systems are running well and there is no compliance or certification in your industry, then you should be able to carry on using them, as long as you ensure they are kept secure and clean from infection - and that means good permieter protection and ensuring any network devices which can see the server are also kept clean.