If Trying to Enable
Follow The Link You and I Have Providedhttp://www.comodo.com/secure-dns/switch/windows_xp.html
If Trying to Disable
At Step 5 Click "Obtain DNS Server address Automatically" > Ok > Ok > go to Start > Programs > Accessories > Command Prompt > 'ipconfig /flushdns' (Without '') > close the window. (Or Reboot)
Also what browser are you using?