Is this crash normal? My Snow Leopard crashed while I was switching from Itunes to the browser.

Basically everything got stuck and a the current song second started to run in loop.

Is that normal ? Can it happen very rarely ? Or should I worry ?

thanks
You should worry if it happens to you periodically. If it happened only once, my guess is one of the background process Mac OS X runs from time to time got stuck for a moment.

BTW, when you say "crash", did you have to reboot your computer? Or did it just got stuck for a period of time?

Anyway, if it crashed, the best you could do is send the crash report to Apple when prompted and check the Usual Suspects:  


Background processes => System Preferences > Account > Login Items (remove the one you don't need.
Check how much free space you have left and start removing the files you don't need
Maybe try to reindex your hard drive