It's easy to blame these companies, and there are certainly shady security companies out there (think the freecleanpc.com commercials and such). Certainly, malware isn't created by the most reputable companies, such as Norton and McAfee. There may be fringe companies that dabble on both sides, but I doubt it.
As an IT Professional who places an emphasis on security, I would say that the only way to safely clean your computer is to back up your personal data, and completely restore the PC to a factory condition with the actual Windows install CD from Microsoft if you have one, or a factory restore CD from the manufacturer. Install all of the Windows updates and security patches. Then install an antivirus program. Microsoft offers free Security Essentials, which isn't too bad. There is also AVG or Avast if you would prefer something different. Then you can reinstall your software and copy your personal user files back onto the system.
Many people recommend some utility or procedure that will remove malware from a computer without that step, but it is very difficult (if not impossible) to ensure 100% that your computer is completely clean and safe. It sounds like a lot of work, but it really shouldn't take more than an evening or two to get everything back to where it was before.