search for custom ringtone by chevron wp7 .
the instruction is in this thread iinm .

follow the steps below:-

1.  download  chevronWP7 certificate and  chevronWP7 unlocker.

2.  attach  chevronWP7 certificate to an email and send to you WP7 email account.

3.  download the email attachemnt(do not save the certificate). execute the certificate in the email.

4.  connect you phone to the desktop/laptop. and turn on Zune.

5.  go to c:\windows\system32\drivers\etc and search for a file called "host". right click the files and select properties. uncheck "read only"

6.  Make sure Zune and your phone are connected then run the  chevronWP7 unlocker.

7.  check the 2 box and click on Unlock.

try to disbale Zune auto start when phone connected. or else the next time you connected ya phone to that desktop/laptop. it will lock again. so u have to repeat steps 1 - 7 again to unlock.
to sideload, make sure you donwload windows phone 7 SDK first.