right, the easiest way to do it is via bluetooth. Fire up the bluetooth on your computer, and get it searching for devices. then go into your phone's bluetooth settings and choose 'find me'. hopefully your computer will detect your phone. It may or may not ask you to set up a PIN to enable communication, if it does just type any 4 digit combo you like (you'll then be asked by your phone to enter the same PIN).
Once that's done, open up windows explorer and you'll see something like 'my bluetooth places'. open it up and it may well pause for a bit as it builds a list of devices. with any luck your phone will be listed there, double-click on it, and you'll see a list of 'services' your phone provides, there should be two called 'OBEX....'.
Now open up a seperate windows explorer window and find your mp3s, right-click on an mp3 file and HOLD that mouse button down and drag the file onto one of the OBEX icons in the other window. release the mouse button and you'll get a choice to copy or something similar. select that and then it will be transferred to your phone! The only thing I can't remember is what OBEX icon you need to drag the mp3 file to (as one can be used for transferring pictures, mp3s etc. and the other for contact information), so if the first one you try doesn't work then try the other one.
Hope this helps a bit.