I'm no expert in MacBooks or iMacs (Windows user ftw), but first things first.
Have you tried installing CCleaner and cleaning up any junk left over? You can get it on OS X and Windows.
Have you got any files on there taking a lot of space? Consider backing them up somewhere secure to relclam space.
Try defragging the hard drive. Sometimes, no matter how much junk you clean, the hard drive will still run slowly. Defragging it should work.
Or if all else fails, do a complete reinstallation of Windows 7 and/or OS X