jno's method can also be time consuming if you have all your tracks organised into different folders, eg albums and artists. If you right click on any folder and select properties it will count all files and folders in that folder. So if you have all your music folders in one master folder you can go to properties on that master folder and it will tell you the total files and total folders you have within that folder, eg 7,748 files, 417 folders (is what my music folder counts). The down side to this is that it will obviously count any file, not just music. Things like album art images are often found in album folders if you or your music player has downloaded them (Windows Media Player hides these files).
Alternatively, Windows Media Player and iTunes (and others media players) will provide a count of all music tracks currently in their respective libraries. Note though that just because a music file is on your computer it won't neccessarily be in a media player's library.