You said that you are worried that your notebook might be infected in addition to worrying about your iTunes/Apple account being compromised. That, in combination with the fact that I always assume the worst will be the basis/context for my answer to you.
I always assume the worst as I stated so here is what I would do:
1) Wipe your notebook; if you suspect any kind of compromise or any kind of malicious infection at all then your safest bet is to wipe it and start over. Are there other options here such as trying different scans to try to "potentially" find the compromise? Sure. Could it take forever, and possibly not yield fruitful results? More than likely.
2) Change your iTunes/AppleID password right away. Create another string of randomly generated characters that include the usual: uppers, lowers, special characters, and numbers. May I suggest a password manager? If for nothing else, use the password manager to generate a random password as most of them have this capability built-in to them now.
3) Update your security questions on the account to completely different questions and answers than before.
4) Turn on Apple's two-factor authentication. This is purely OPTIONAL, however it's a great additional layer of security nonetheless.
5) Practice safe browsing and computing habits going forward. I know this is pretty broad and generalized, but the more you're aware of what sites and services you visit and use and their practices as well as their reputation while online, or even offline for that matter, the safer you'll stay in the long run.
These are just the general overview items. Please let me know if you'd like any more in-depth.