I managed to install 4.1.21 into my easyphp 1.8 and it works fine. The only thing to look out for was the installation path. I had to set it to c:\easyphp1.8\mysql
See the thread for a screen shot...
http://moodle.org/mod/forum/discuss.php?d=56304#258931258931
Installing and upgrading help
Moodle 1.6 and Easy PhP 1.8
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