You can't be certain your system is malware-free unless you reformat and reinstall -- and you'll get a superclean PC in the process Read More ..
In my last column, I talked about making online shopping safer, starting with ensuring your computer isn't already infected with some devious malware. But I didn't tell readers how to confirm that their computer wasn't maliciously compromised from the start.
Let me give it a shot. First, the reality is that without extreme measures (such as comparing every file on your computer to the vendor's known, legitimate checksum), you can't have absolute assurance that your computer is malware-free. If you want that, format your computer's hard drive and reinstall everything from vendor-distributed media and content -- then disable the network card and never connect to the Internet.
Unreasonable advice aside, here's how to determine with some degree of assurance that your computer is malware-free, even after you've surfed the Internet. This column contains the steps I take when I try to verify my own computers (or those of my friends or neighbors) aren't infected.