New iPod + account not showing up on a computer that was previously used with a DIFFERENT iPod and account

Question

I'm very new to all things Apple, but not a computer/tech newbie...it's just that all my experience and knowledge has been aquired with Windows and Android. I received an iPod Touch 5th gen as a Xmas gift, and I'm (mostly) enjoying it...but I've hit the following problem:
About a year ago, a friend used iTunes on my computer (WinXP Pro) using HIS account/Apple ID, and connected HIS iPod to my PC to "sync" (I'm not sure what he was trying to accomplish, but my computer and his iPod were definitely conected via cable). He no longer needs to use my computer to access iTunes.
So, now I want to use MY iPod and iTunes account with the same computer that he used. My friend and I "authorized" my computer using my Apple ID, DE-authorized it using his, we signed him out, signed me in, (not necessarily n that order, heheh). BUT... in my iTunes library still show HIS apps, HIS books, and his iTunes-U courses. None of my apps/books/courses etc. appear in iTunes on my computer. (They ARE all on my iPod). I tried un-installing iTunes and re-installing the latest version, and that did not solve the problem.
I have heard or read, somewhere, that if I connect my iPod to a compter that has someone else's iTunes account on it, my iPod could be un-intentionally "sync'd" to that persons account, and I would lose my apps/music/etc and wind up with the other persons "stuff" on my device (I'm sure there are some subtleties to that that I'm clueless about). My hunch is that the solution lies somewhere in connecting MY new iPod to my computer, but I'm afraid of losing what I've already put on my iPod and/or messing up my friend's devices.
I appreciated you taking the time to read this; I'm hopeful the solution is simple...thanks!

Answer 1

Hi lllaass, thanks for your reply. Here are my results, and related follow-up questions, so far:
lllaass wrote:
- Make sure your computer is authorized for your iTunes account.
OK, done, AND DE-authorized for his account (tried this even before I posted).
lllaass wrote:
- You said your friend no longer uses your computer. Therefore just delete his media *apps, music etc) for the computer
When I highlight one of his apps (for example) and select DELETE, a pop-up window opens, warning me that the app will ALSO be deleted from any device "which syncronizes with your iTunes library". How can I be certain that this is not affecting his stuff? Is that ONLY possible if he connects one of his devices to my computer via cable? Is there NO chance that it happens via wi-fi (we share the same wi-fi connection regularly), or even via the cloud?
lllaass wrote:
- Redownlod your media to your iTunes library.
Downloading past purchases from the App Store, iBookstore, and iTunes Store
I followed the instructions in the link above (computer authorized, signed in under my account, & "Show iTunes in the Cloud Purchases" enabled in iTunes preferences by going to iTunes > Preferences under the Store tab).  However,  none of MY "purchases" (all free apps, books, etc, so far) show up in the library...only my friend's. For what it's worth, when I try to open any of his books, a pop-up opens telling me that the book "can't be viewed using this computer. You can view this book using iBooks on your iOS device".
Illaass wrote:
-Then just sync to as necessary to your iPod.
- However, yo may have to backup the iPod to your computer and restore from that backup if the iPod sees the computer/library as a new iTunes library. The instructions are here:
Syncing to a "New" Computer or replacing a "crashed" Hard Drive: Apple Support Communities
With iTunes 11 you may have to do a Control+B to show the Menu bar and a Control+S to show the Sidebar.
I have not tried these steps, given the status of the situation being unchanged so far.
Thanks again Illaass, hopefully there is another approach to try.