Vista - Out of memory, WMP11, any media app

Hello! Before anyone telling me to upgrade my ram here is my computer specs:

Core 2 Duo E6750
4GB Ram Corsair XMS2
Windows Vista Ultimate X64
500GB HDD x 2.
Geforce 8800GTX 768MB

I've tried searching the forums, found some information but NOTHING helped me. I can't play any MP3, DVD or any media file on my computer, be it with Windows Media Player, Itunes, Winamp, Realplayer or Nero, they give me different problems, Realplayer just don't play it, WMP and Winamp gives me errors about out of memory and Nero says it couldn't open the file.

I haven't installed any additional codecs other than ffdshow and ac3! My other partition with Windows XP (Which doesn't detect all my 4gb) runs everything just fine, I have about 38% of memory used only, I've tried rebooting, other players and all but nothing. My computer still plays games though, including ones that works with mp3s like GTA-SA.

Any help?

Thank you