Did you expect that simply reading a webpage would make mIRC behave differently?
Reading the page should inform you about the new behaviour in mIRC regarding codepage support-- specifically the lack of support for codepages. Greek characters work fine in mIRC 7.1, but you need to use Unicode, not codepages, to display them. This should be fine on your end, but your friends should start to switch over to unicode. Most other clients already have, just tell them to enable UTF-8 in their mIRC options.
As far as the nickserv message goes, you should inform the server administrators that your IRC client does not support the text encoding used by their services and request that they pick a more platform neutral encoding (utf-8) with which to send their messages-- or otherwise start supporting other encodings.