which browser did you use? which website did you get this problem?
It sure sounds like a security problem of the browser (if you indeed get infected through the website), the malicious codes getting to your messenger through executable code on website.
1. turn off "open safe files after download"
2. be careful not to visit suspecious websites.
Safari doesn't remind users about malicious websites when they are trying to visit them. If you visit a lot of different websites, be careful. It doesn't hurt to try out firefox 3 beta4 if you want. Who has probably better measures on this topic.
What do you mean "adium won't work"? the app is broken and won't start? or it doesn't meet your need?