Recognising certificates

OK this is quite a complicated one... I am trying to access a Windows based network remotely. The network is a secure server run by the school where I teach. The problem is when I try to access the network from home on my Mac, it tells me the certificate for the network is outdated. I can open the network home page, but the main frame shows only encrypted information. I can see some of the page, including a link to download the certificate, which I did, and put it in my keychain in both the login section and the X509 Anchors section (I wasn't sure which it should be in). There was already a certificate there from the school network provider, which was outdated, so I deleted it. The new certificate is well in date. I cleared my cache and history in Explorer (it doesn't work at all with Safari) but I still get a message saying the certificate has expired and an encrypted page! The network manager and technician at school have no idea what the problem is, but knew someone else with a Mac, who had had the same problem and had solved it by selecting 'do not save encrypted pages to disk' in Internet Options/Advanced Tab. There is no Internet Options...is there?? If she means Preferences, there is no such checkbox under any tab in Safari, Explorer or Firefox. Oh, one other complicating factor... I did check some boxes in Preferences that looked as if they might make a difference, and I deleted the up-to date certificate from my keychain so I could start over again with a newly downloaded copy, but when I try to download it again, I get a message that I've already got it so it doesn't let me download. It isn't on the list in keychain access, though. The network itself is accessible without problems for Windows users - will I just have to buy a Windows PC to use this facility?