I have a feeling that it is to do with the network we have but one of tyhe useres on my computer is a limited account called ASP.net, which you cant log on to. What does it do?
I believe it is the account that is used to execute ASP.net code on your computer. Similar to the IUSR_<machine name> that gets created when you use IIS.
That is exactly what it is (it's actually "ASPNET" as the account name). It is the impersonated "user" account that ASP.NET websites run under when executing code.