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No Sound For Computer

What's happened is my computer won't play any sound; it just suddenly stopped yesterday night, and was working compltely fine before. There's not reason for it to have stopped, and I don't download any dubious stuff and anything I do download gets checked by my MacAfee anti-virus.

It's NOT the sound being muted, and it's not me accidentally turning the sound to zero; I've tried turning it up, watching videos on youtube, no sound, twiddling with the volume a bit, trying again, and alas, no change. i tried Windows Media Player, which plays no sound, and through my headphones for both sound from the internet and sound from WMP; both produced nothing on both the computer's normal speakers and the headphones. (I know the headphones AND the computer's inbuilt speakers both work because I tried the 'StickyKeys' thing where you press shift 5 times and it makes a noise and it made a noise, and my headphones work fine with my iPod so it's something with the computer).

Could anyone give me some advice on how to fix this? My computer is made by Acer, and it's a Windows XP

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