How to implement Sandboxie

As I saw Sandb开发者_运维知识库oxie makes a virtual space on hard disk and there are allowed the programs to write.

This how can be implemented as software? Which windows (kernel,shell ?) functions needed to be overriden?

Software like Sandboxie basically provide a virtual execution environment for (sandboxed) applications. They do this by virualizing file system and Registry (read/write/delete/exec operations) among other things. They are also called feather-weight virtual machines, as they provide a virtual machine like environment for individual applications. You can refer to these pages for more info:
http://sourceforge.net/projects/fvm-rni/ (open source app)
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.74.1367&rep=rep1&type=pdf (paper)
http://www.ecsl.cs.sunysb.edu/tr/TR224.pdf (paper)

I don't know how Sandboxie does it, but the usual way is through File System Filter Drivers:

A file system filter driver intercepts requests targeted at a file system or another file system filter driver. By intercepting the request before it reaches its intended target, the filter driver can extend or replace functionality provided by the original target of the request.