Der Befehl
\usepackage[english,greek]{babel}
lässt MikTeX 2.9 ewig laufen.
Betriebssystem Windows 7
Die Ausgabe ist
This is pdfTeX, Version 3.1415926-2.4-1.40.13 (MiKTeX 2.9)
entering extended mode
(C:/Users/x/AppData/Local/Temp/lyx_tmpdir.Hp3100/lyx_tmpbuf2/negreponti-1.tex
LaTeX2e <2011/06/27>
Babel <v3.8m> and hyphenation patterns for english, afrikaans, ancientgreek, ar
abic, armenian, assamese, basque, bengali, bokmal, bulgarian, catalan, coptic,
croatian, czech, danish, dutch, esperanto, estonian, farsi, finnish, french, ga
lician, german, greek, gujarati, hindi, hungarian, icelandic, indonesian, inter
lingua, irish, italian, kannada, kurmanji, latin, latvian, lithuanian, malayala
m, marathi, mongolian, mongolianlmc, monogreek, ngerman, nynorsk, oriya, panjab
i, pinyin, polish, portuguese, romanian, russian, sanskrit, serbian, slovak, sl
ovenian, spanish, swedish, swissgerman, tamil, telugu, turkish, turkmen, ukengl
ish, ukrainian, uppersorbian, usenglishmax, welsh, loaded.
Bearbeitet:aus dem Kommentar des OP, kleiner Code:
\makeatletter \makeatother
\documentclass[11pt,oneside]{amsart}
\usepackage[iso-8859-7]{inputenc}
\usepackage[a4paper]{geometry}
\usepackage{amsthm}
\usepackage{amstext}
\usepackage{amssymb}
\makeatletter
\usepackage[english,greek]{babel}
%\usepackage[default]{gfsneohellenic}
%\usepackage[LGR,T1]{fontenc} %% LGR encoding is needed for gfsneohellenic
\makeatother
\begin{document} klkk \greektext γφγγ
\end{document}