I run Vista Home Premium with iTunes 184.108.40.206 for media.
All was fine until relatively recently when I started regularly getting an error about iTunes being unable to save its library file because of not enough memory. BUT I have plenty of memory. At the time this error pops up (and I have a Vista gadget showing me memory utilisation real-time) I have 850MB free - it doesn't vary - it's not as if it dips iTunes shows an error and then it recovers, it permanently shows around this much free (I have 2GB total) when iTunes is running.
I've done a web search but cannot find evidence of anyone solving this problem (other than getting advice about getting more memory!)