Your iPad can only be synced with one computer at a time. If you try to sync it with a new computer/iTunes library, it will replace the contents of the iPod with whatever is in the new library. You should probably disable the autosync functionality first by going to Edit -> Preferences, clicking the Devices tab, and enabling the prevent iPods,iPhones, and iPads from automatically syncing option.
Before doing anything else, authorize the new computer with your iTunes Account. In iTunes, choose Store -> Authorize This Computer and enter in the correct credentials. Either copy a backup file from your old computer to your new one or create a new backup of your iPad in iTunes before letting it sync.
Then right->click on your iPad from under the Devices section in the left hand pane of iTunes and choose Backup. You might also want to take a look at this article to see what it all included in the backup.
Now onto synced content such as music, videos, photos, etc. For iTunes purchases you can copy them back into iTunes by choosing File -> Transfer Purchases. For all other nonpurchased iTunes content, see this excellent user tip from another forum member turingtest2 outlining the different methods and software available to help you copy content from your iPad back to your PC and into iTunes.
Once the backup has been made and all other synced content such as music, videos, and photos are back in your iTunes library, restore your iPad from that backup you made earlier. Here is more on backing up and restoring your iPad.
If you do happen to lose any purchased content, you can always redownload it from the iTunes Store again at no cost via iCloud. See this article for information.