Are there any restrictions set in Settings/General/Restrictions?
Is the sync process stopping right at step one (backup) or does it go right to the end and nothing is changed on the ipod?
Is your iTunes authorized with the same account as your ipod? About iTunes Store authorization and deauthorization
Did you check your security software settings? iTunes for Windows: Troubleshooting security software issues
If there was an error code that would help.
Try turning off all the categories so it is down to minimum.
Disable synching bookmarks, books, movies, music videos, etc.
I had to delete the Photo cache, iPod Cache, that gets created and is hidden, in some folders.
I don't know if there is a certain amount to leave free but suspect like any filesystem, it needs room to write new temp files, rename and delete. I ran into trouble with auto synch after purchasing some TV shows - I really think "Auto Sync" is or can be hazardous.
Also, I needed to recharge and plugged into a computer other than its "home" and though I thought I did the safe thing, I started having trouble after. Same with plugging in and a different computer user account was active, it is also married to a single user account (two user accounts on two computers so anyone can sit down and work was never this much trouble).
There are a number of Apple tech support articles. Not sure if you can find all of them, or that they are linked to from product support home page (they should be):
if it stops at step 2, delete the latest backup listed in iTunes/Edit/Preferences/Devices and try again.
Back up the backup folder before, in case you'll need it again, info about the location on your HD can be found inthis article: iTunes: About iOS backups