The Dell Latitude D600 had a Pentium M CPU and that's not very fast. Fedora would be slow on it, but it's still usable. My laptop has a Pentium M underclocked and it works fine.
It depends on how much memory you have. I'd recommend
Mint with the Mate desktop, if you have 512MB
ZevenOS with Xfce will run in 192MB
ZorinOS Light will run in 128MB
These are all beginner-friendly and they all allow you to encrypt your /home partition, which is a good idea with a laptop (although you could always put a label on it: "Don't steal me, I'm old"!)