It sounds like you may have a rootkit in your system which is impossible to detect with today's software anti-virus tools. In fact, rootkits are so bad that they run "underneath" your anti-virus and prevent it from even seeing it. They can read and write memory stored in flash or firmware, as well. the only way to rid yourself of a rootkit, safely, is to basically reformat your entire drive and reinstall the operating system and your software - basically starting from scratch.
if you own a business, today you are forced to notify people of any breach in security (customers, students, etc) that have information on that computer (salary, SSN, address, name, etc). this costs $200 per contact. Legislation is also being changed to allow companies to be sued for this breach of data if negligent practices are in place. All data must be encrypted. I would suggest that if you are in the process of getting a new PC, you should considern these two new PC functions:
Secure Harddrive from Seagate:
http://www.eetimes.com/news/latest/show ... =197802341
Intel Trusted Execution Technology:
http://www.zdnet.com.au/news/hardware/s ... 313,00.htm