No worries. It is really difficult to have something done which breaks your system on this matter:
From KDE menu - Configuration - KDE - System - Access Manager.
There you'll setup KDM as default Login Manager;
After a logout/login check and if everything is OK, you could erase (if you want, although it is a small program, so you shouldn't even bother) GDM (GnomeDisplayManager)
4.- Check linmodems.org, as Winmodems like you (and mine, shit!) are not always recognized.
The problem with the hardware support on linux comes from the manufacturers not publishing them specs. Almost half of the linux drivers are made by "guessing" the specs of the hardware !!!!
Apart, there are evil-examples such as winmodems, which use chips with hardcoded windows routines which prevent from using it outside windows.
The day MS falls to blackmail manufacturers and these begin total support for linux, that day the Total Linux World Domination will arrive :-)