Convert p7b file to CER/PEM/CRT with microsoft gui tools

 Doubleclick the p7b file to open it, expand all the folders.  In the list of certificates, you might see multiple certificates, as p7b files can be a collection of certificates, which often include the full chain of certificates up to the root.  When converting to a CER, PEM, or CRT file, we are making a file with one certificate in it, so you need to select the specific cert you want to create a file for.  In this example, I'm using fiddler's certificate

Double click the cert you want

Click the install certificate button and use the user store.  Change the option to place certificate in a specific folder and select personal from the popup window

Once the certificate is installed, click start->run-> certmgr.msc

Expand the personal->certificates folder.  Right click on the certificate that you just imported, select all tasks -> export

Use the do not export the private key option

Select the DER or Base-64 option.  This will depend on what system you are using and what it supports.  Base-64 is probably the safest option.

Select an export folder and filename for the converted certificate file.