It is fairly easy but I suggest making a backup of your important data first. It can always go wrong.
Steps from within Windows (so this will -not- be detailed):
- Shrink your Windows partition with something like Partition Magic;
- Leave the space unallocated;
- Download the Ubuntu ISO for your system specs(32bit or 64bit);
- Burn it to a DVD (it wont fit on a CD);
Steps after this are not related to Windows:
- Reboot (with the option in BIOS set to boot from DVD) with the DVD inserted;
- It will end with a desktop and a popup. Choose "install" to start the installation. Answer all the questions (it will ask for locales, username etc) and, when asked, install Ubuntu in the unallocated space (it will ask for what to do with the installation and this will be an option).
And that is basically it.
It IS very very very easy to do. But you do need a bit of experience in dealing with it but if you keep your eyes open and read what is shown you will be fine.