The windows software I create in TideSDK, does it run in sandbox or can I have full access to file system?
The answer is yes and no..
If you are loading the page which is from local hdd / filesystem you will have full access to the filesystem through window.Ti object. you can see the documentation of various apis available here
Secondly if you are loading any remote server url .. you will not have access to any of extended apis as the "window.Ti" object is not available.