Have you tried a different web browser to determine if it's a problem with the browser (even though it only affects those two sites, a browser-specific problem is still a possibility). That possibility needs eliminating before looking elsewhere for the cause. The main alternatives to Internet Explorer (assuming that's what you currently use) are:
Firefox - here: http://www.mozilla-e...oducts/firefox/
Opera - here: http://www.opera.com/
Netscape - here: http://browser.netscape.com/
Another possibility is that somehow those two web addresses have been added to the Window's HOSTS file in a manner which stops them from being found.
The HOSTS file can be used for this purpose when you wish to prevent access to malicious or unsavoury websites, but some malware can also put perfectly legitimate websites in there just to frustrate you --- the Windows Update website is a common victim of this.
The HOSTS file is here on Windows XP: C:\Windows\System32\drivers\etc\
It's simply called 'hosts' or 'HOSTS' (there's no 3-letter extension at the end), and it can be opened, edited and saved with Notepad (right-click, 'Open with...')
If you see in there the two problem web addresses with the numbers 127.0.0.1 at the beginning of each line, delete those two lines and save the file.