Dell ins1525 with vista is not shutting down.

Hello,
ani here,happy new yr to all of you, i have a big problm with dell inspiron 1525 shutting down.i surfing net regularly by dial up connections via bluetooth.but recently,my laptop is not shutting down in proper way,sometimes it taken more than one hours and sometimes it does nt shut down,it happens generally after net surfing... plz give me any suggetion as early as possible....thank you.