Amazingly, one of the most popular links on this site is the quick tip, Getting free diskspace in python. One of the comments shows that this method doesn't work on Windows. Here's a version that does:

import win32file

def freespace(p):
    """
    Returns the number of free bytes on the drive that ``p`` is on
    """
    secsPerClus, bytesPerSec, nFreeClus, totClus = win32file.GetDiskFreeSpace(p)
    return secsPerClus * bytesPerSec * nFreeClus

The win32file module is part of the pywin32 extension module.