xampp was freshly installed and no customisation in httpd.conf yet
1. edit c:\xampp\apache\conf\httpd.conf and add the following at the bottom of the file
# Virtual directory settings
Include "conf/extra/httpd-virtualdir.conf"
2. ensure that the following is present and not commented
LoadModule alias_module modules/mod_alias.so
3. edit c:\xampp\apache\conf\extra\httpd-virtualdir.conf and add the following to create the virtualdir /abc
<IfModule alias_module>
Alias /abc "c:/someparentdir/somedir"
<Directory "c:/someparentdir/somedir">
Options Indexes FollowSymLinks Includes ExecCGI
Order allow,deny
Allow from all
Require all granted
</Directory>