OSX 10.5.6 blowing up left and right

Damn it, MacOSX blows the hell up when I:

• Tried to download software using Safari. I tried downloading with Firefox and it seems fine.
• Running Adium_1.3.2.dmg. However, the crash only happens when I DOWNLOADED Adium on OSX, and then attempted to run the .dmg. In fact, if I downloaded it on Windows, and then run it on OSX, the Adium seems perfectly fine.

So I guess as of now, the moral is NOT using my MacOSX to download stuff from the browser. BUt then, how to System Update worked? I did twice a while ago and they all work great.