See:
http://devwiki.pfsense.org/BootTroubleShooting
http://devwiki.pfsense.org/BootOptions
You should also try downloading the FreeBSD 6.2 (i386) ISO and see if that produces the same problem. If it does then it's likely that your hardware (whatever it is - you still haven't said) doesn't work with pfSense/FreeBSD.