This one has my ISP helpline stumped - any ideas? Please help!

Hello to everyone, I am sorry if this seems a dumb question but I promise I've done everything I can think of and it's driving me crazy! I use Windows XP home and all of a sudden I am unable to access my webmail at webmail.blueyonder.co.uk I can get to the log-in screen okay, but when I enter my details I just get the standard Windows "page not found" error. At first I thought the site was down, but I checked the status page and it said it was okay. I called my ISP's helpline and they said webmail was fine. They said another user had just phoned in with the same problem and used ZoneAlarm, and she found when she reinstalled that, all was fine. So I tried uninstalling and reinstalling ZoneAlarm - no joy. I even tried it when ZoneAlarm had been uninstalled completely just briefly, and it still didn't work, so I know it's not ZoneAlarm. I tried uninstalling and reinstalling AVG Anti-Virus Free too, and again this didn't help and even with both ZoneAlarm and AVG uninstalled, I no joy. I tried a different browser (I was using IE6) but both Firefox and Opera just give me a blank page - it doesn't come back as page not found, it just gives me a blank page and acts like it's loaded it. No other site is affected - other webmail sites are fine, and secure sites are fine too. The helpline suggested I restore Internet Explorer settings to default, no help. I've tried adding the site to my trusted zone, turning off IE's pop-up blocker, no joy. I tried System Restore but it didn't help, and when I tried to go back earlier it just said it didn't work. I tried a few dates but that didn't work either - and when I tried undoing my last restoration it said it couldn't do it! I've tried this on another PC and I can access my webmail, so I know it's an issue to do with my PC, but I can't think what. I haven't changed anything and I just can't understand what would prevent me accessing just one specific site across three different browsers! I even tried entering the wrong password and it takes me to the standard "wrong password" screen and lets me go back, so it's just the actual mail it won't let me access. My ISP has admitted defeat but says as it's obviously a setting on my PC, they can't help me any further. Any ideas would be really appreciated as I just don't know what's going on here! It seems such an odd fault!