Please don't blame me if downloading this skin makes your computer explode or anything, I will not be held responsible. Anyway, it shouldn't ought to, and I'll only end up blaming your computer.
This skin is designed to work with Version 5 of messenger. If you try it with an earlier version, then all that will probably happen is that the button pictures won't work, and you'll be left with the defaults instead.
Also, you'll need Winzip in order to unzip the files once you've downloaded them. If you haven't got it, you can probably get it from here if you search around hard enough.
Right, on to the actual instructions...
Download the winzip file to anywhere on your computer (but make sure you know where, because you'll have to be able to find it again in a minute!)
In Windows Explorer (or wherever!) wander round your computer and find where Yahoo Messenger is. eg. on mine it's in c:\program files\yahoo!\messenger\
Once you've found this, there ought to be a folder called 'Skins'. Create a new folder within the Skins folder, and call it 45s (or whatever you want really, it's not important... as long as you remember what it's called)
Go back to your winzip file that you downloaded. Open it in winzip. Extract all of the files to the folder that you've just created.
In messenger, open up the preferences window (Login menu, then select Preferences). Click on 'Appearance' from the list on the left hand side. Near the top there should be a drop down list labelled 'Current Theme:'. The name that you gave to the folder that you created (eg, 45s) should now appear in the list. Select that name, than click ok at the bottom of the preferences window.
Viola! You should now have your new 45s messenger theme!
If you've done all of this and it hasn't worked, or you got halfway through the instructions and thought "eh? what's she on about?!", then please email me and I'll try and sort you out! :)