The browser could not find the host server for the provided address.

[sen]

Hello everyone maybe I post in wrong session but I'm really in trouble. I got a problem while trying to access the internet webs. I mean certain webs like grisoft.com and avast.com
can't be accessed. It also interrupted my access while trying to downloading any files. The errors messages from mozilla firefox
is shown below;

The browser could not find the host server for the provided address.

* Did you make a mistake when typing the domain? (e.g. "ww.mozilla.org" instead of "www.mozilla.org")
* Are you certain this domain address exists? Its registration may have expired.
* Are you unable to browse other sites? Check your network connection and DNS server settings.
* Is your computer or network protected by a firewall or proxy? Incorrect settings can interfere with Web browsing.

But others general webs like google.com and yahoo.com showed no problem for me to get in. It just like I need a license for avast.com
and grisoft.com to enter while others seem don't have to.

What should I do to fix it so I can access all webs normally. I'm a windows Xp Professional Service Pack 3 user.
comments would really help me.

[Reedbeta]

If you are connecting to the internet via a router, try rebooting it. Also, try doing 'ipconfig /flushdns' in the command line. If those don't fix it, it might be spyware (seems likely if the websites you're having trouble getting to are all antivirus websites.) Lastly, it might be a problem with your ISP.