If you have XP then it is a more suitable OS for DAW, however I would still recommend tuning it.
This link is the source used by most when it comes to setting your PC up for use as a DAW.
I would recommend you read it carefully as it does point out things that can do harm and what not. If you follow its basic principal well you should have a good system for doing music on.
This may not solve your problem through so I would have to suggest that you follow Onkel Donkel's advice and try use other forums to find the cause of the problem. Are you sure there is nothing running in the background - check the processes running in Windows Task Manager - you may find that you have WMP running in the background.
I am still convinced that something else is hogging your soundcard when Cubase loads! That is the only time I have ever seen the message you described.
Drammy