When you are ready to abord on the flight, in airport you have to claim your new computer in Canadian custom. If you are not, when you back to canada, you need to pay for tax. If you have invoice, it's better to take the invoice with you.
Any thing regarding to children porn, you need to delete if you have anythings.
If you have free downloading software installed in your computer, it will be ok. Just make sure, having any illegal copy of software(CD) with you, you might have trouble such as Windonw XP, vista and so on, if canadian custome officer find them. Any illegal copy(CD) is a big deal, so you might take chance.