The forward slash translates correctly in windows...that's not the problem.
The problem you have here is you're getting c:\c:\new_1.htm
I found the easiest way to work with the template class is by setting a default path in the constructor for Template and having sub dirs for different pages as appropriate:
$t = new Template; ## e.g. default = c:\test\
$t -> set_file ("ack", "page1/body.ihtml");
will load c:\test\page1\body.ihtml into template var ack.
There were several bugs in the template class for phplib releases prior to the current one. Make sure you use the most recent phplib for a good template script.