You can convert mhtml to pcx by using HTML Converter Command Line which allows you to convert web pages to PDF, PS (postscript), WMF, EMF vector formats.HTML Converter Command Line also allows you to create previews or thumbnails of web pages in a number of different formats, such as TIF, TIFF, JPG, JPEG, GIF, PNG, BMP, PCX, TGA, JP2 (JPEG2000), PNM, etc.
You can download HTML Converter Command Line at https://www.verypdf.com/htmltools/htmltools.zip. If you want to know more about HTML Converter Command Line,you can visit https://www.verypdf.com/htmltools/htmltools-manual.htm to read the User Manual of it.
When you first download HTML Converter Command Line,it shows as a zip file.You must unzip it to use.But you will find there are limitations to the functions.To solve this problem,you need to register HTML Converter Command Line in cmd.exe dialog box with the code you get when you buy HTML Converter Command Line.
The following contents are the details of converting mhtml to pcx file format by HTML Converter Command Line,may you can get some useful information here.
Step1.Please run cmd.exe.
You can click 
button first and click “run” in start menu.Then input “cmd” in “open” edit box of the popup dialog box and click “OK” button.Please see Figure1.
Figure1.
Step2.Input commands.
Please input command lines in cmd.exe dialog box.Before inputting,if the interface of cmd is disordered,you can input the command “cls”,then click “Enter” button on your keyboard,which means clear the screen.Please see Figure2.
Figure2.
The whole command lines to convert mhtml to pcx in the dialog box are "C:\Documents and Settings\admin\Local Settings\Temp\htmltools\htmltools.exe" "C:\Documents and Settings\admin\Desktop\Google.mht" google.pcx
"C:\Documents and Settings\admin\Local Settings\Temp\htmltools\htmltools.exe" stands for the path of htmltools.exe which is the called program in the conversion.
"C:\Documents and Settings\admin\Desktop\Google.mht" stands for the path of mhtml file format.The .mht in the directory is the extension name of mhtml file.
google.pcx stands for the name of target file which includes file name and extension name.The target file will be saved in current directory where the whole conversion runs by default.
Step3.Browse target file
You can find the target file in location C:\Documents and Settings\admin and you can open or browse the file by some kind of viewer such as GSview and IrfanView,etc.Please see Figure3 and Figure4.
Figure3.
Figure4.