The problem of my imac is described here:
http://docs.info.apple.com/article.html?artnum=93988

I have OS X 10.3.9 and my last option was to contact apple. B/c I bought it on Oct'04 the technical support by phone is over (90 days) and I was asked to pay 49usd if I wnated to have the problem analyzed.

I haven't solved the problem but at least I know is not related with Limewire.