Your best option is to use my Debian/Ubuntu package from http://cmeerw.org/debian/
Although PyXPCOM packages can be downloaded from ActiveState's Web-site, I don't recommend doing it because the distribution files there are outdated. Better get the current version checked out from the Mozilla CVS repository (see mozilla.org: source code via cvs. You only have to login if you haven't already done so (the password for user anonymous is anonymous):
cvs -d :pserver:anonymous@cvs-mirror.mozilla.org:/cvsroot login (Logging in to anonymous@cvs-mirror.mozilla.org) CVS password:
Then change to the directory where your Mozilla sources are and check it out by issuing:
cvs -d :pserver:anonymous@cvs-mirror.mozilla.org:/cvsroot checkout -rMOZILLA_1_7_BRANCH mozilla/extensions/python/xpcom
Before continuing you should probably check the Python related settings in extensions/python/xpcom/src/Makefile.in and extensions/python/xpcom/src/loader/Makefile.in to make sure that they match your setup.
To have the PyXPCOM makefiles integrated into Mozilla's build system, you will also have to apply this patch in Mozilla's home directory: pyxpcom-allmakefiles.diff (via ftp (it is based on this bug report).
In order to build PyXPCOM you have to create the makefiles by running configure in Mozilla's home directoy with --enable-extensions=python/xpcom first. Then you can build it by simply typing make in PyXPCOM's directory:
./configure --enable-modules=xpconnect --enable-extensions=python/xpcom --disable-debug --enable-optimizecd extensions/python/xpcom && make