Sounds odd....I have a 2007 white MB and it runs pretty well...although I replaced HD with a much larger one a year ago or so...
Is everything you do on it slow or just some activities ( ie, websurfing, mail, videos, etc...)?
If slowness relates to using programs involving graphics or video, etc, then possibly your frustration has to do with MB graphics chip...don't think there's much you can do about that...one compromise that MB's have vs MBP's is the graphics chip is not as good...
Did you install lots of programs on it? Or use some programs that require better machines to work well (ie, video editing, CAD, 3D modeling, etc...) ?
If none of the above fits, then maybe it would be wise to bring your MB in to your local Apple store (if one available) or local Mac or PC repair shop to have them take a look....might help....