change windows system path from cygwin python?

[gry at ll.mit.edu]

The _winreg api looks helpful; unfortunately, I'm trying to stick to
what can be got
from the cygwin install -- no _winreg.  Simplicity of installation is
quite important.
(I'm using cygwin to get the xfree86 X-server, which is the whole point
of this exercise)

I have found the cygwin command-line "regtool" for munging the
registry, so I plan
to use that via os.popen.
Thanks all for pointing me to the right place in the registry!