If all four browsers do it, the problem is either with the DNS server set up in your PC or the ditto at your ISP. If you can't get to bom.gov.au but that link works, that narrows it down to a DNS problem.
Open a command window and type ipconfig /all
You'll get a load of stuff, among which will be the DNS server address. If you're using a router, it may give the router address (same string as DHCP server and Default Gateway). In that case, disconnect the power from the router for a minute or two and reconnect. That might shake the router into updating itself.
If you get a DNS server address that's not your router, then look it up (if you know how) to see what it is. Otherwise, post it here and I'll take a look.
The other possibility is that something has played around with the "personal" DNS file in your PC. I can never remember the filename/location, but it's a place where you can put a specific IP address to use for certain URLs - someone'll come along in a minute and tell us the name :) If those addresses of yours are in there, with a wrong DNS, then you'll get the effect you're noticing.
EDIT: found it - c:\windows\system32\drivers\etc\hosts - with no filename suffix.
Open that with Notepad (or whatever you prefer) and check that there are no addresses in there for the ones you're having trouble with.
Last edited by Keef; 17th December 2006 at 00:41.