you went from mac to pc? :o
just kiding. pcs are good for some stuff over macs. but that isnt the point
macs accel in ease of use. that being said, sometimes drag and drop on mac isnt as easy as drag and drop on pc.
manually enter the files into your itunes. ctrl+o brings up a browsing menu, and locate the files that you want to add to your library. hold down shift to select file x, y and everything listed in between (varies depending on how the file is arranged, name, date, etc.) or control for multiple ones that arent in order. if i were you, id move all of the files into a folder (which should already be conveniently done) and just do the shift trick.