How do I save my credit rating?

[Jelsoft]

In the event I have to reformat my hard drive, how do I save my credits in emule?

[cpugeniusmv]

you probably can't unless someone knows where the registry entry(ies) and/or file(s) that store that data are.

it's not worth the trouble.

[zaphodiv]

Your credit in emule is not stored on your computer. Your credit is stored on the computers of people you have uploaded to. Your identity on the emule network is stored in userhash.dat. If you keep the same usershash on your new computer you will be given credit by people you have uploaded to previously.

Saving the credit information that your client has built up about other people may allow you to get ti-for-tat sharing going a bit faster/

I suggest saving all the *.met, *.dat and *.ini files from the emule directory and taking the opportunity to upgrade to the latest version. Copy the files back to the emule directory after installing the program, overwrite any fresh files that the installer creates.
You could just saving the emule directory from program files.

Apart from the ed2k:// link assocation, emule does not use the registry as far as I am aware.

[DainBramaged]

Zaphodiv is correct. Save the files he mentioned, install the new copy of eMule, and copy and overwrite the files back into the new installation. In this way your credit rating is preserved.