Did you check your Router, I hear some of those can be infected?
If you're using Wifi/Airport, what kind of Encryption is it using?
If Verify Disk reports a problem, try this, but it's not very likely related to the DNS Changer problem...
Does it boot to Single User Mode, CMD+s keys at bootup, if so try...
/sbin/fsck -fy
Repeat until it shows no errors fixed.
(Space between fsck AND -fy important).
Resolve startup issues and perform disk maintenance with Disk Utility and fsck...
http://docs.info.apple.com/article.html?artnum=106214
Apple no longer has or sells 10.4 Discs with slight exceptions if your model came with a Gray Tiger/10.4.x Disc.
See Tom's, (Texas Mac Man), great info on where/how to find/get Tiger...
https://discussions.apple.com/thread/3089008?answerId=15305521022#15305521022#15305521
Or Ali Brown's great info on where/how to find/get Tiger...
http://discussions.apple.com/thread.jspa?messageID=10381710#10381710