help needed really badly please please please //(*_*)\\

hi all im new to using pcs, i was talking to some1 on her called gwendolin, i was told it was an idea to upgrade my pc because i wanted to download movies and i was having problems,i only have a 256 on my pc, so i went to pc world and they gave me my little board for my pc so know i have 512 i think know on my pc i added another 256. my problem is now i have no sound? at all i cant play cds i cant hear pc sounds and all i keep getting is an error windows cannot play the sound %systemroot%\media windows xp battery critical.wav. your sound card maybe in use . please can some1 help me.