easiest way:
take the 40 gig drive out of the old computer, make it a slave drive, install it in the new computer. instant transfer.
a little more advanced:
if you have lack of room for drives, disconnect one of the secondary drives in the new computer so you have a slot for the 40gig (such as cd drive). Hook up the 40 gig, start up and then just copy the files from the 40 gig to a drive that has enough space to hold the files. Then shut down, disconnect the 40, hook back up the other drive that was where you ran the 40 (put back the cd drives connections) and boot back up.
precautions:
remember that whenever you disconnect a drive, you want to pull the power plug out of the tower first, and if you computer wont boot back up after installing the new drive, make sure you reconnected the power.