This is a known problem with iTunes and 64bit OSes. I got it when I had XP x64. I looked into it and found that out. There is some kinds fix that didn't work for me. XP x64 gave me too much trouble so I gave up on it and went back to XP 32bit.
32 bit supports only 3.25GB of RAM, that means if you have 4GB, 768MB is not found or something... If you have like 4GB or more of RAM, you should get a 64bit OS.