I copied this from another discussion. If you follow these instructions you should be OK. Here is the link to the discussion as well.
Here is another discussion - dealing with a Mac - it works the same way on a PC or a Mac - only the iTunes terminology is a little different.
When you set up the iPad without using iTunes and then you try to sync to iTunes - your computer sees the iPad as a new device. You now have to associate the iPad with your iTunes library.
1) Without connecting your iPad to your laptop, start iTunes. Click on iTunes. Click on Preferences. Click on Devices. Check the box next to "Prevent your iPod etc. from automatically syncing." Click OK.
2) Now connect your iPad to your Mac and start iTunes.
3) When iTunes starts, right click on your iPad under Devices in the left column. Select Transfer purchases etc.
4) After it finishes transferring all your apps to your computer, right click on your iPad and select Backup your iPad.
5) After it finishes backing up your iPad, right click on your iPad and select Restore etc.
6) After it finishes restoring, left click on your iPad , then click on the Apps tab on top, and check the box next to Sync Apps, then click on Apply below.
If everything on your iPad looks good after the sync, go back and click on iTunes / Preferences / Devices and UN-check the box next to Prevent your iPod etc. The only other thing you may want to check is if your contacts, bookmarks, etc. are syncing correctly now. If not, go to the Info tab after connecting and make sure you indicate which features you want to sync with what sources.
That should do it.