The mystery of iTunes

Can anyone explain it?

I made a momentous decision yesterday and took my nine-year old desktop PC to the recycling centre (after doing all the necessary clearing of files, caches, etc).

Everything from there was transferred to a portable hard drive, including my vast iTunes library - although I hadn't added to it for a while because I knew this move had to be made.

When it came to opening iTunes on my new laptop and downloading all the tunes, and also synching my iPhone with the new device (eek I've only got one device left to sync with now) I discovered that any of the tunes I had bought via iTunes on my iPhone were not backed up and have not appeared in my iTunes library on my home laptop.

Now as far as I'm concerned I bought the damn tracks and they should be in my iTunes account regardless of where I bought them from.

Can anyone explain to me what is going on? Am I doing something wrong?