Problems with iTunes creating MP3 version for all songs?

I’m trying to create the mp3 version for all my songs in iTunes.

I select all songs, then select create mp3 version.

It will go on fine for a few minutes then iTunes will crash, giving me a message error. I can exit and restart the program just fine.

Does it matter what format the songs are in to begin with if I want to create the mp3 version? I think they’re a mishmash of formats right now. Do I need to select them one by one?

I’m using Windows Vista 64 bit and iTunes 10.

How do I get iTunes to stop crashing when creating mp3 version?