The sites are blocked in your computer then. May be a symptom of malware infection. Scan with a good antivirus software first and post the Hijackthis log here.
There are various reason for this.
U may try Setting DNS
Go to control panel network connection right click on local area network properties internet protocol (TCP/IP) properties select use the following DNS server addresses
^^Avira is a very good antivirus and is available from http://www.freeav.com/ . There is a free version but it doesn't includes malware protection as far as i know. Use Avira along with Spybot Search and Destroy available at http://www.safer-networking.org and/or ad aware from http://www.lavasoft.com to get full protection under Windows XP. If you are running windows Vista Genuine copy, then update the definitions of Windows Defender first using windows update and do a through scan. If that doesn't works then download and use the software i previously mentioned.