I installed 3.0rc1 on a Win XP 2.4Gzh system with all current updates
with no problems. Cmd line Python and docs work fine. 

Tried to start IDLE but got error message: "IDLE's subprocess didn't
make connection. Either IDLE can't start subprocess or personal firewall
is blocking." 

I turned off my firewall but got the same error message. IDLE works fine
in Python 2.5 and 2.6 on this same machine. Tried change to
idlelib\run.py suggested in bug report #3905 but it had no effect.

...Bob

It is probably already corrected in svn.

Can you please try from a command prompt:

> cd <path_to_python3.0rc1>
> python Lib/idlelib/idle.py

Does this display some error message?

I think the problem is fixed.
See issue 3905.

But #3905 is not yet fixed, furthermore the OP already tried the
correction suggested there.

Amaury, I did as you suggested - running idle.py directly both from a
command line and by clicking on it and the results were the same. Each
time, I got the same error message window as before. In addition,
several seconds before the error window came up I got the following
traceback in the command window:

C:\Python30>python Lib/idlelib/idle.py
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Python30\lib\idlelib\run.py", line 76, in main
    sockthread.set_daemon(True)
AttributeError: 'Thread' object has no attribute 'set_daemon'

I hope this helps.
...Bob

Did you *really* follow the suggested change proposed in #3905 ?
http://bugs.python.org/msg73496

Amaury, my stupid! I must have forgotten to delete the run.pyc file
before trying out the modified run.py as suggested in #3905 . It all
works now and IDLE starts up just fine.

Sorry and, thanks for the help.

...Bob