Hmm.
Is the iTunes library present on the computer; all you songs etc?
If so, the below should work. If not, ask, and we'll try and find your library. Or, if you don't have anything in iTunes on the computer that is not already on the iPad, the below should still work.
Go to the Store menu and authorize the machine with the Apple ID you use with the iPad. It probably lost that authorization when you updated. It would have been best if you had deauthorized iTunes before upgrading, but you have 5 authorizations available, so it shouldn't be a problem unless you need more computer's to be authorized with that account.
Go to the iTunes Preferences. In the Devices tab disable automatic syncing. This keeps thing from happening until you are ready. You can re-enable it later if you want auto syncing.
No connect your iPad to the Computer with iTunes running. When/if it comes up with the erase dialog, cancel it. Then right click on the iPad under the Devices list and choose Transfer Purchases.
This will both make sure that all your iStore purchases on the iPad (but not on the computer yet) get copied. It also associates the iPad with iTunes so that it will stop trying to erase your iPad.
Now, this is very
important.
Go through all your sync settings and make sure they are what you want them to be.
Pay extra attention to the Apps section. Make certain every app you have on the iPad is checked for syncing.
If you do not, the next sync will remove those apps and any data they contain. Content like music, books, whatever can easily be corrected later. Documents can be lost forever.
Which means it is also a good idea to copy any documents or pictures especially important to you off the iPad before doing any of this.
When you have the sync settings to your liking, and double checked, go ahead and click the sync/apply button.
Good luck.
Edit: Found this Apple support link:
http://support.apple.com/kb/HT1848
You may prefer it to my off the cuff procedure.
