1. Purchase a complete antivirus/malware package such as Mcafee, Norton, AVG if this is practical for you to do or if you have a package already re-install it and update it as malware may have damaged it to the point of non-function. There are trusted websites that carry viruses as a result of hacker activity and many sites with "FREE" stuff that tend to give you a bonus with your download or even run malware as a part of their coding.
2. If you have little or no protection then download some free ones to clean the system, I personally use...
AVG Free available from http://www.grisoft.com (You can also purchase a full edition)
Malware Bytes anti Malware Free available from http://www.malwarebytes.org/ (You can also purchase a full edition)
Super anti spyware available free from http://www.superantispyware.com/ (You can also purchase a full edition)
There are many other software options available, choose the "Flavour" that suits you.
Once installed run superantispyware after it updates and reboot as requested, rerun the software after the reboot and once completed run the malware bytes anti malware, reboot as requested and re-run after reboot.
Also make sure your antivirus has updated and scan with that also but run these scans separately as running them together can cause problems.
I run these 3 items on a few of my computers with good success but as always, a full commercial antivirus/spyware/malware package is the best option and subscribed users get 24/7 phone support.
Best Regards - MM
