When using Skype Internet Telephony the computer freezes and must be manually powered-off, as CTRL-ALT-DEL does not work. This same problem occurs when using Yahoo Messenger. The computer is running Windows XP using an ADSL connection through a D-Link DSL-200 ADSL modem.

As always with such problems, our first procedure should be to reinstall the programs in case any files have become corrupted. Make sure you backup any important data in both programs before proceeding, as I am unsure if any data will be lost when uninstalling, and then reinstalling, these programs. In the likely situation that reinstalling the programs does not help the situation, we should have a look at any firewall settings which could be causing difficulties. If any firewalls are not allowing Skype and Yahoo! Messenger full access to the services they require, it could cause the programs to freeze. Since you are connected to the internet using the DSL-200 ADSL modem, this problem would not be related to incorrect port forwarding or blocked ports on the ADSL modem since it is just a modem and not a router. So, our first point of call is the Windows XP Firewall. If your computer is running Windows XP Service Pack 2, the inbuilt Windows Firewall could be blocking Skype. For instructions on configuring the Windows Firewall to work with Skype, visit www.skype.com/help/guides/firewall_winxpsp2.html. Likewise, this could also be the cause of Yahoo! Messenger freezing. In this case, follow the same instructions given at the Skype website, except add Yahoo! Messenger to the list of approved programs. Of course, if you are running a third-party firewall such as ZoneAlarm, make sure the firewall is configured to allow inbound and outbound traffic for Skype and Yahoo! Messenger.

Secondly, try reinstalling the sound drivers on your computer. Many users experience problems when running Skype due to corrupt or incorrect sound drivers, and this may contribute to the problem with Yahoo! Messenger if you are using the voice chat capabilities. Finally, the problem could also be related to missing or corrupted Windows files. Windows XP includes a handy utility which will check all system files and recover new versions of any corrupted files. Go to the ‘Start’ menu > ‘Run’ and type ’sfc /scannow’ (without the quotes) and click OK. The System File Checker will begin. During the procedure, it may prompt you for the Windows XP CD, if the information is not already on your hard drive. Once the SFC has completed, it will exit silently and not give you any feedback.