Reinstalling OS

My computer crashed and I had to reinstall the operating system Windows XP.

I also reinstalled iTunes (which was also lost) and I tired to synch my iPhone to the computer.

iTunes asked me to authorise the computer, which I did, now I have two computers

authorized (I guess the operating system that crashed and the newly installed one).

I went through "synch", the program instructed me to "transfer all applications" and this is

where all my purchased applciations were lost.

How can I get he "apps" back into the iPhone and backed up in the new iTunes???

Thank a million.