Sounds to me that you could also be experiencing any viruses/malware/spyware that could block certain sites.
Do you have any programs that you can use to scan ?

Also, try downloading mozilla firefox (free browser) at http://www.mozilla.com/en-US/firefox/ and see if you get the same problem.
If you're using IE, look to see if you haven't blocked certain sites in your settings. I'm assuming you're running XP.

Also, you can clear your cache in your browser.

Hope this helps.
Claude