If you remember the settings somewhat from before, it is better to simply set them back like you would configure Plus! in the normal way (Plus! > Preferences & Options).
I say this because you said you don't have any scripts or skins.
Extracting the other standard registry options from the backup can be a daunting task and is prone to errors.
You also CAN NOT simply import them since many registry settings include path names (which are different from OS to OS), and other settings might depend on other OS stuff...
So, if you don't have scripts or skins you realy shouldn't bother tampering with those registry settings and instead simply configure Plus! again via the normal way, via its configuration panel...
In that way you come across all its options again and learn or be reminded what they do (you would be surprised how many people don't know that some options even exist because they never bothered to look in all the panels).
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PS: In case you had custom sounds, they were (by default) stored in C:\Documents and Settings\All Users\Application Data\Messenger Plus!\Custom Sounds...
Simply back up all the files (.DATs) you'll find there and put them again in the current Custom Sounds directory on your new OS.
After that you can run the
Sound Library Fixer to add the information of those sounds again to the Plus! Library.
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