You need to download the LIP (language interface pack) to use on any version of Windows 7 that gets about 80% of the system into a particular language, but unfortunately it seems the LIP is not available for Dutch. To have a full Dutch installation you would need to be running Win 7 Ultimate or Enterprise, which have a full Multilingual User Interface.
If you wish to gain multi-lingual support for Windows without risk, you will need to buy Ultimate or Enterprise.
Depending on what you use your computer for, you may find that Linux Mint can fufil your needs, Linux Mint is free and unrestricted by langauge, but may not run all your WIndows applications (alternatives are avalible). http://www.linuxmint.com/