Say that you had a suspiciously slow morning, ran a scan at noon and successfully removed a trojan. However, in those few morning hours, the trojan may have had downloaded a rootkit that is very difficult to detect and remove. Can you ever trust the machine again? Is malware removal worth the effort? Or would you rather rebuild the machine (reinstall the OS and programs from original disks, and files from backup) after say every bit of adware that you are getting?