Sound not working? Please help!

i just reformatted my computer. i am using xp.
but for some reason my sound isnt working
i really dont know why? i have the speakers powered on and plugged in correctly but sound isnt working through my internet browsers or in Windows Media Player (havnt downloaded anything else yet) i get the message:

"Windows Media Player cannot play the file because there is a problem with your sound device. There might not be a sound device installed on your computer, it might be in use by another program, or it might not be functioning properly."

i believe its a codec/driver problem. but i really dont know
someone please help!

well i ran a few of those Auto Detecting driver programs and they both updated a few, but still no sound...
its a Dell Dimension E521(i know, i hate it too)
what are the sound drivers for that? and where do i get them?

thanks for the thought... but Dell are assholes and expect me to pay for another warranty before they help out someone. ive called in before and all they say is "your warranty is expired, you can pay to renew it, otherwise we cant do anything about it"

same with the site, i tried it and it said please renew your warranty.