Net on mobile via bluetooth with pc

Hello,
i have sony ericsson k800i , i want to browse net on it via bluetooth in my computer .. i tried many times , it did conncet to the net , but when i tried to sign in to messenger , ( for instance ) it failed !! ; it only worked with net on my sim-card , not with net in my pc ! how can i do this ?
nore: when i select '' net over bluetooth '' in my mobile , it asks me for a username and password , then it asks me to select between '' CS '' or '' PS '' .. what are all those ? plz help .. thank you so much ..