Best way to "overlay" Ubuntu over existing Windows OS

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http://askubuntu.com – I have an existing windows installation with important applications. I want to take advantage of unix tools and OS environment of Ubuntu. What would be the best way for me to 'overlay' Ubuntu on top of existing windows environment? - I need to run both OS and use the windows apps everyday (the windows apps are computationally intensive like Matlab--i'll migrate these apps to use the linux version one by one once I see the setup is better for the way I work) - I would like to have ubuntu to see the windows file system (Windows XP, windows Vista, windows 7 for three diff computers) - I would lik (HowTos)