Is there a way for me to get my songs back? (iTunes)?

I recently got an iPod touch. Never had one before, never had to deal with iTunes before.

I made my first purchases over the past couple of days. 2 songs that I like. They were listed on my iPod and on my computer in the iTunes library.

I decided I wanted to add some music from my own CDs just tonight and picked a particular song from a CD. When I opened iTunes, I did think it strange that some of my stuff wasn't showing up (only an album I uploaded yesterday was showing), but being new to iTunes, I figured I just didn't understand how it all worked. I added the song I wanted and went to check on my iPod. The songs I had purchased are gone. From iTunes and from the iPod, even though they were on my iPod earlier today.

Is there some way for me to get my songs back without having to purchase them again? And is there something I'm supposed to do to backup or something so this won't happen again? (Btw, I'm on Windows XP.)