Installing GalnetMIUI Rom on HTC Desire
Notes:


Rooting allows you to install a custom recovery (Clockwork Mod or AmonRa) to install ROMs
S-OFF allows you to change the HBOOT to rearrange your internal partitions


I rooted with Unrevoked before I used Alpharev's S-OFF some months later

Nowadays it's easier apparently. You can get S-OFF with Unrevoked Forever too but I'm not sure your phone is supported. And Unrevoked/Alpharev teamed up for revolutionary

Finally, there are many ROMS out there, some are better then others. I use Oxygen personally with the Oxygen HBOOT for 330MB internal data space. I'd use Devnull (same developers) but it doesn't have long press to kill