Updates iPhone 4

Where is your music, if the answer is "on my device, or it was" that's the wrong answer.

Your music should be on the computer, you must put it there, it does not magically appear because you signed in with your ID or synced your device.

Also, sign out of iTunes with the other users Apple ID and sign in with your own.

Just FYI, when acquring a second hand computer, it's generally a good idea to restore the computer to factory settings.  You'd had to find out the original owner left some spyware or keylogger software on the computer after they destroyed your financial life or got information to blackmail you with.

Thanks but no thanks there is no music and all of my paid for apps are gone and how on earth am I able to sign into iTunes with this guys ID when I have no clue what his password is. Oh nd FYI the laptops fine thanks

And you are absolutely positive there is no keylogger software on it? How do you know that?

The problem is you left the former owners iTunes library on the computer and you iPhone synced with his data (which at some point it probably warned you that your iPhone had not been previously synced with this computer).

Connect your iPhone again and Restore it as a NEW iphone (not from backup).

Next you need to delete the iTunes folder (not the iTunes library folder, the one above that under music in your account) and either replace it with yours from your backup of your previous computer or just start iTunes and create a blank one then Restore your iPhone in iTunes. You can download any iTunes purchased music and apps again.