That isn't necessarily a symptom of the Flashback. What other symptoms are you experiencing? It sounds like your mail account may have been hacked or spoofed. Change the password immediately and be sure you aren't using the same password for other things.
Is your 10.5.8 running on an Intel or a PPC Mac? The Flashback isn't known to attack PPC.
Here is a reliable detection and removal tool, at least as of 4/12.
Thank you for your answer. Actually my Mac has got an Intel processor. But so far I have not detected other symptoms apart from a failure to install an update. It said my volume isn`t big enough (which is strange). I had the same password for my e-mail account and for my Mac administration
I ran the tool which you recommended and it said. "clean of flashback"
When you do a Get Info on your Boot Drive, how much Free Space does it say it has?