preventing dialer-programs to be install

There are only two ways I'm aware of that lets a phone-dialer programs install on your PC, the first is if it's included in a program you download & install (usually a program from some little known company that includes it to make money).

The second is the more likely, your surfing along & open a link & all of a sudden you have several other windows open up.
Say 3 windows openned, now you try to close them by clicking on the X in the upper right corner, this works for 2 of them, but the third only displays a hand when you move your mouse over the X, DON'T CLICK THIS ONE.
There are several ways to close this nasty window, which might install a dialer app or run some other script.

Before you do anything else open another instance ofIE, which should open to your homepage (if more windows continue to open either disconnect from the Internet or if running a firewall such as ZoneAlarm put the lock on), now do a CTRL+ALT+DELETE & select the one instance of IE that represents the window that won't close & END TASK it, you may get a not responding dialogue box, if so end that also.

This should have closed the window & maybe others depending on how many you had open, but the new instance of IE I had you open should remain. Thereason I had you open that window is because when you do the END TASK without openning a new window, you would likely loose your taskbar. That isn't a big deal all you would need to do if that happens is double click your My Computer icon & the taskbar will reappear.

If you didn't open a new instance of IE & END TASK the other window you would loose your taskbar but when you tried to get it back most of your icons would be gone, rebooting is the only way to get them back.