Couple of points:

32bit is correct if you aren't sure, and an older system is more
likely to be 32bit anyway.

The download is an ISO file and needs to be burned in a special way to
create a bootable CD. Double click on the ISO file and see what
happens. This needs to be done on the PC you downloaded the ISO file
onto, and you can test if it's been done correctly by ejecting the CD
and then closing the tray again - you should get the Ubuntu loader
screen offering something called a WUBI install, this is a way of dual
booting a Windows PC with Ubuntu. If all you get is the ISO file in an
explorer window then it's just been copied to the CD.

Stick with it and I'm sure you'll get there :-)