If you can open sites by using the IP, there may be a problem with the Hosts file:
Open it and delete the lines containing the addresses that you cannot visit.
Also check the Restricted Sites list in Internet Explorer:
Click the Tools menu
Click Internet Options...
Click the Security tab
Click Restricted sites
Click the [Sites...] button
Remove the web sites from the list
If you still cannot resolve the problem, use System Restore to restore Windows XP to the previous working state.
Backup your system before starting.
Click Help and Support
Click Undo changes to your computer with System Restore