The following are some of the best programs we have found for protecting your computer system from harmful software - they are also free and will work with most versions of Windows including Vista and 7.
1) Free Anti-Virus software
Anti virus software is essential for your system and the 4 suggestions here are all free for personal use and better than most of the commercial products you have to pay for.
a) AVG-Free is an excellent product and can be downloaded here
b) AVast! is another excellent product and can be downloaded here
c) Avira yet another excellent product and can be downloaded here
d) Microsoft Security Essentials - excellent product - download here
2) Free Anti Malware software