I've found a more consistent pattern. It only happens when I am actually downloading using Limewire (there are no problems with the connection - up to 70KB/sec).

Usually it happens when I try to open a movie file (any type), but it has happened with just Outlook Express. The temporary fix (which works every time) is to simply disconnect Limewire when I want to open a new application. It does seem like a conflict between the graphics card and the modem since Limewire can safely be open, just not downloading. I will try (when I work out how!) to roll back my graphics drivers from the omega ones to the latest nVidia ones and see if that helps.

Although the fix isn't ideal, I can live with it. If anyone has any other ideas I would love to hear them.

Thanks for the help
Ross