Hi, I have an iPod that I bought off somebody a wile ago his computer was the first computer to be connected with the iPod. I put on a passlock onto my iPod and my niece changed the code and forgot it.. is there any way I could unlock the iPod ?

The only way to unlock it is to connect it to that first machine.

The other option is to restore it with your machine. But you will loose all your data...

You firstly have to put the iPod into disk mode. Here is the instructions:
Putting iPod into Disk Mode (http://support.apple.com/kb/HT1363)

When in disk mode plug it into the computer. Itunes will then give you the option to restore.

Done :)

Use iTunes to reinitialize the iPod then you should be good to go