DailyTech reports Windows 7 will be the most secure Windows operating system to date, and that especially the XP Mode in Windows 7 is a great security asset because it gives many types of malware no chance.
Windows 7's security is thanks in part to the OS patching routes to inject malicious code into the memory. In previous Windows OS's, such as Vista, memory protections such as DEP and ASLR offered a degree of safety. However, there were routes to get around these protections, relatively easily. With Windows 7 blocking many of these routes and additionally with applications such as IE8, Firefox 3 and their plug-ins (Flash, Acrobat Reader, and QuickTime) at last utilizing these protections, Windows 7 is shaping up to be a very strong fort.
Additionally, Windows 7's XP Mode is likely to cause more headaches for attackers. XP Mode is implemented using hardware virtualization extensions. A common hacker tool -- rootkits -- rely on hardware virtualization and a special privilege level called VMX root mode. With the OS now using hardware virtualization, attempts to gain the privileges necessary to launch the special hardware virtualization support needed by the rootkit tends to crash the OS or provide the user with warnings. For this reason Blue Pill, one common rootkit, doesn't work well in Windows 7.