Sandisk M240 Deleting Problem

Is there a way to erase ALL the songs in one click on the player?

I bought it (my first mp3 player), put it on my computer and put a few songs on it. For some reason, all of my songs are on the player twice!

So that's not too good. I go into Windows Media Player and it asks me to sync the device. This sounds like a good plan so I do so. Then, without asking me, it finds EVERY SONG ON MY COMPUTER and puts it onto the device.

I've found that I can delete any of the songs that I manually imput into the device, but half of the songs on the player don't show up in the Windows Media Player menu.

This is very frustrating as these songs (which, for some reason, there is three of some now ) are taking up valuable storage space. I can now only fit about 5 CDs on my 1GB player because most CDs are on twice, if not 3 times!

I can only delete a few of the items- so is there a way to completely reboot the machine and erase it's history?