HELP!! My iPhone won't play my music because it doesn't sync correctly.

I bought my new iPhone 3GS a week ago. I had a little trouble syncing music, but got it to go once. After that i tried syncing again and it didn't work. What happens is it shows that the music is in the phone, but my iPhone does not show that there's music in iTunes. Everything else syncs: Apps, Ringtones, calendars, e-mail, contacts, etc. I get an error message several times per sync stating that the "iPhone cannot be synced; the disk cannot be read to or written from". I am using a PC laptop to sync. I've tried reinstalling iTunes three times, restarting my computer and iPhone, everything I looked up online.
I think it may be my computer that is the problem, because i took my iPhone to a mac retailer, and they were able to sync music from their Macbooks, no problem.
I talked to an apple specialist on the phone for an hour yesterday, and he didn't know what the problem was. Can anyone help me?