NSS Labs, the leading independent security product testing and certification organization, has examined six major browsers- Apple Safari, Google Chrome, Internet Explorer 7, Internet Explorer 8, Mozilla Firefox, and Opera against socially engineered Malware and published a report called ‘Web Browser Security - Socially Engineered Malware Protection’.
The report reveals that Internet Explorer 8 (RC1) is the safest one with a 69% catch rate against Malware. It was 30% in Mozilla Firefox (3.07), 24% in Apple Safari, 16% in Google Chrome (v1.0.1549) 5% in Opera v9.64, and 4% in Internet Explorer 7. (You can download the complete test result from here (pdf))
From this report you can clearly understand that presently Internet Explorer 8 is the safest browser that you can rely on. At the same time Internet Explorer 7 failed to perform a better result.