what is your browser?
Sounds like your ISP or connection, how do you connect?
DNS Issue, try configuring your DNS Manually.
I recommend Open DNS, it is free and it’s fast http://www.opendns.com/
Oh, the reason you are having this problem now and not before is likely you have the ISP DNS Hardcoded somewhere and your ISP changed the DNS IP Addresses and you have auto-forward DNS Disabled.
Long Story short, the easiest way for someone like me to tell you how to fix, is to config a manual DNS.
oh, one other thing, the reason you can get to the initial pages is that the ip address for these sites are cached in your browser and on the computer.
Are you using a firewall?
I always start by assuming that I have a virus. Get ccleaner from www.ccleaner.com. Run that. Then, over on the left hand side click on registry and run that program. If you are still having trouble download antivirus Avira from http://www.free-av.com/en/download/1/avira_antivir_personal__free_antivirus.html
Make sure you get the free version and run it.
Clear your cache and temp Internet files. Everything.
Then install Firefox. Experiment with it and it will become more comfortable than IE. More features and free addons too.
Super Antispyware. Recommended to me by Microsoft tech support when I had malware turn off my Windows updates.
www.superantispyware.com
CCleaner is also worth having. Dump IE and go to Firefox..get the free plugin Adblock Pro.