Standard RS-232 modem driver / RAS troubles

Hi

Is the standard serial (RS-232) modem driver anymore provided by Microsoft
for the Windows Vista OS? I cannot find such driver for Vista Business
(Control panel -> Modems or Vista installation media).

I've tried the standard modem driver from XP, but it seems not to be
reliable.

=> RAS/dialup connection goes often into state that it produces H/W failure
and wont recover (modem query works always).

Also installing another serial modem driver to same COM port replaces the
current modem linked into RAS connection, so there seems to be some sort of
mixup?

Regards,
Erkki