when I play music using media palyer, it comes message:cant play since there is a problem with sound device. I checked device manager, everything is working properly. I even removed the sound card and uninstalled driver then put it back, but still same.

when I try to open volume control, it shows a message:there are no active mixer device available. I thought it maybe some spyware is using my sound device since I only hear some radio noise when I touch the speaker. So I did Norton and Adaware, and didnt find any virus or trojan.

Then I mumma and google Internet, I found some posts about exact same problem, but there is no answer on solving the problem, the only suggestion is system restore or reinstall XP.

Also some other problems:
1. When I try to enable XP ICF(Internet Connection Firewall), it says: the dependency service or group failed to start. Maybe I stopped some service? I played service.msc a bit.
2. I can not enable the switch user option, only can log off.

So I hope my writing is making sense to you, and I could get some help.

Thanks.