Most of font that come with Windows 7  rendered a thai tone mark incorrect.
I test with font Tahoma ,Times new roman ,Sergoe UI,Arial all have same issue.
But some font that i download from internet and install later is rendered correct eg. "TH Saraban PSK".
You can download "TH Saraban PSK" font from http://nms.rmutp.ac.th/wp-content/uploads/2010/10/13Font-Files.zip
            
I test with font Tahoma ,Times new roman ,Sergoe UI,Arial all have same issue.
But some font that i download from internet and install later is rendered correct eg. "TH Saraban PSK".
You can download "TH Saraban PSK" font from http://nms.rmutp.ac.th/wp-content/uploads/2010/10/13Font-Files.zip
 P4
  P4