The speakers attached to my computer are built into the monitor and mysteriously the sound works in the morning but does not in the afternoon. This problem has been around for about 18 months and reinstalling Windows XP has not fixed the issue. Do you have any suggestions?

This is certainly an interesting question since the problem does not appear to be software related as reinstalling Windows does not fix the problem. Before arriving at a potential cause for this problem we should complete some additional steps to troubleshoot the issue further, particularly whether the problem is with the speakers or the sound card in the computer. When the sound stops working in the afternoon it would be interesting to plug either a different set of speakers, or perhaps headphones, into the speaker jack on the sound card and check whether they work. If they do work correctly then the problem appears to be hardware related with the speakers within the monitor. In this case a potential explanation for the issue could be a component inside the monitor which stops working after several hours of the monitor being active and speakers being powered-on. In any case, in this situation you will likely need to purchase a new set of speakers for the computer. It may also be a good idea to get the monitor checked in case this problem is indicative of a more serious fault with the monitor (which could have safety implications).

However, if the sound does not work with another set of speakers (or headphones) connected to the computer then it seems that something is probably wrong with the sound card in the computer. In this situation I would recommend taking the computer to a technician for evaluation. They should have the capacity to determine whether the sound card is actually the cause of the problem (by swapping a known-good sound card into the computer to check whether the problem occurs) and then replace the card if necessary. Alternatively, if the sound card is not the problem then they should be able to investigate other possible causes of the issue.