In the spirit of stupid - a stupid question:
If Linux is loaded onto an operating Windows machine, the resulting arrangement is dual-boot. When the pc is booted into Linux at the boot selection menu, is browsing with the Linux OS - in every way as secure as if, there was no Windows partition?
Or is it still somehow possible for Windows to be exploited?
No. If Windows isn't loaded there is no hint of Windows on the machine when Linux is running. Only if the Winfdows kernel is loaded can Windows vulnerabilities be exploited.