oooh-la la
Try at your work PC to disable all cookies and then try and use the site. Often your session information can be stored in the URL instead and not as a cookie. When you first access the site your http request will inform the web engine that your browser does not accept cookies and if it can, it will try to store your session info in the URL.
If this works ok at your work PC, then go home and do the same. At least at this point you know for sure that something on your PC is interfering with your cookies.
Alternatively download an http trace tool and post the output you get on this thread.
Kill your firewall and try. Maybe you have an http proxy running on your PC and it has an exclude list that you never knew about.
Or you could try buying a decent motorbike, give up on those BMWs (riding a BMW is like sitting in an armchair which is balanced on a pogo stick in a bowl of custard). I'm sure you would not get the same problems on a Triumph site.