Win98 Internet Explorer 6 web site restrictions on broadband

[Mr T 2007]

Hi all I have a very strange problem: I am running win98 (first edition) and internet explorer 6 on a PC. After having no problems for a few years, I suddenly find that I now cannot access certain web sites using Orange broadband. I can access Google, two of my bank accounts, the Answer Bank, and several other sites, but I cannot access hotmail, yahoo, my creditcard sites, my main bank account, uk street map and many more - the web pages will not open. However, if I switch from broadband to dial up, then all web pages can be accessed, and the problem has gone! I have connected my Orange broadband modem to my Mac, and the Mac can access all sites with no problems, so I'm pretty sure that it is not my broadband modem that is at fault. If I go to "internet options" - "security", I cannot adjust any settings, as the "custom levels" and Default Levels" are greyed out, is this normal? Could this be part of the problem? Also, I am running AVG free edition anti-virus, there seems to be a limited amount of settings on it, but even if I quit the whole programme the problem is still there. I upgraded from IE 5.5 to IE 6 to see if this would solve the problem, and also tried using Firefox web browser, but the problem is still there. I tried accessing hotmail through the megaproxy web site, and (after a short while) it came up with this message: "JavaScript required to sign in Windows Live ID requires JavaScript to sign in. This web browser either does not support JavaScript, or scripts are being blocked. To find out whether your browser supports JavaScript, or to allow scripts, see the browser's online help." Sadly, this doesn't mean anything to me, as my technicle knowledge is not great. But is this my problem? If so, what can I do? I would appreciate any help or advice that anyone has to offer.
Many thanks.