Linux will be able to access the Windows partition, by mounting the partition in the file manager, at boot-up, or through the command line, but Windows will not be able to access the Linux partition without the installation of third party software.
There are a number of ways to run Windows programs on a Linux system.
1) Setup your PC as a dual-boot system with both Linux and Windows installed separately, but as you pointed out this means you decide which OS to load into as you boot your system, so it's one or the other.
2) Run your Windows stuff in WINE (Wine Is Not an Emulator), Cedega or Crossover Office.
Non of these will run ALL Windows programs and it can be a bit awkward to get the programs to work 'properly'...If at all...but when it works, it usually works well.
Please note: Cedega and Crossover Office are commercial products, and as such are not free, WINE is free, and it should be available in your package manager, there is also a package called 'PlayOnLinux' which greatly eases the installation of some Windows programs in WINE...The list of programs which work is growing all the time.
Related (option 2) Links:
WINE homepage -
http://www.winehq.org/PlayOnLinux homepage -
http://www.playonlinux.com/en/Cedega homepage -
http://www.cedega.com/Crossover Office homepage -
http://www.codeweavers.com/3) Use Virtualization to run Windows inside Linux (or vice-versa) as a Virtual Machine (VM) this has definite advantages over the other options, as (like option 1) it is 100% compatible with Windows programs as they 'ARE' running in windows, but there is no need to reboot to run the programs.
Please note: very rarely (these days) there may be some hardware issues...USB devices used to cause problems, but seems solved in most recent versions.
It has been reported by some that Windows can run quicker as a VM in Linux than it does natively, but only if your CPU supports 'Hardware Assisted Virtualization'.
Related (option 3) Links:
KVM (Kernel-based Virtual Machine) homepage -
http://www.linux-kvm.org/page/Main_PageVMware homepage -
http://www.vmware.com/VirtualBox homepage -
http://www.virtualbox.org/
