Famous (not here) Manufacturer of antiviruses, BitDefender conducted the following test: collected 385 different viruses, which were especially active in the last 6 months, successfully infecting windows under Windows 7, and tested how the Windows Defender defender will behave in Standard delivery of Windows 8.
The results of the test are very good: only 15.84% of malicious programs worked on Windows 8. From myself I will add that this statistic is inclined to trust: your own short-term experience of using Windows 8 Professional forced to pay attention to the good job of the Windows Defender.
Conclusions from news
Thus, even without an installed anti-virus, the new Windows 8 operating system can be nice to resist some viruses. In addition, I suspect that some percentage of these 385 viruses would be able to "crash" even if there were any of the most common antiviruses - Kaspersky, Nod, Avast, etc.
Message to Windows 8 Defender
If Windows Defender has been disabled, then, according to the company, 234 virus successfully infected the system, the rest did not start, were blocked by the control of user accounts, etc. In general, a good result for the built-in defender. Unfortunately, I did not find the same information for Windows 7, but I will once again note that I can say on personal experience that the defender in Windows 8 monitors what is happening on the computer very carefully.