Thanks to all of you !
I followed Calestar's advice and downloaded
libstdc++6-3.4.1-4mdk.i586.rpm . When I installed it Mandrake said it was something wrong with the file, but it was installed anyway. Then I installed Skype.
Now Skype is working. I haven't tested everything in the program yet, but so far it seems to work fine.
Thanks!