First guess would be some sort of corruption in the existing datastore.
Suggest the following:
First, on the Start/Run line enter "net stop wuauserv"
Next, delete the SoftwareDistribution folder in the Windows (WinNT) folder
Last, Restart the computer, and before doing anything else visit this site: http://update.microsoft.com/microsoftupdate/v6/default.aspx?ln=en (watch
for popup blocker notices and download/install any controls offered).
