Hi

Have you managed to put your hard drive into the caddie you bought?

I am thinking that your new computer is likely to have the SATA type of hard drive connection, this is a small thin connection (about 1.5cm x a couple of millimeters).
Depending how old your old computer was the hard drive was probably an IDE connection which is quiet a bit bigger about three quarters of the width of the hard drive itself.

If this is the case, you won't be able to simply place the old hard drive inside the new computer. However there is no reason why you shouldn't be able to put the old hard drive inside an external caddie and connect it by USB.

Was the caddie you bought a SATA one or an IDE one?

I hope this helps.

Many Thanks

Leo