How to create .pfx file from certificate and private key?
You will need to use openssl.
openssl pkcs12 -export -out domain.name.pfx -inkey domain.name.key -in domain.name.crt
The key file is just a text file with your private key in it.
If you have a root CA and intermediate certs, then include them as well using multiple -in params
openssl pkcs12 -export -out domain.name.pfx -inkey domain.name.key -in domain.name.crt -in intermediate.crt -in rootca.crt
You can install openssl from here: openssl
If you're looking for a Windows GUI, check out DigiCert. I just used this and it was fairly simple.
Under the SSL tab, I first Imported the Certificate. Then once I selected the Certificate I was able to export as a PFX, both with and without a keyfile.
The Microsoft Pvk2Pfx command line utility seems to have the functionality you need:
Pvk2Pfx (Pvk2Pfx.exe) is a command-line tool copies public key and private key information contained in .spc, .cer, and .pvk files to a Personal Information Exchange (.pfx) file.
http://msdn.microsoft.com/en-us/library/windows/hardware/ff550672(v=vs.85).aspx
Note: if you need/want/prefer a C# solution, then you may want to consider using the http://www.bouncycastle.org/ api.