Kat, In the future save yourself a lot of trouble and ditch iTunes. It sucks. Amazon is cheaper, better quality and DRM free. Your OS isnt the problem with converting your music. If you are still having trouble, send me a message and we'll figure it out.
not sure if i heard this right or not, but if you have multiple computers you are allowed a certain amount of (re-downloads) per purchase. not sure how you go about doing it on a different cpu cuz i only have 1