ok, try this:-
1. Hold power button UP for 10 secs (or until screen goes completely dark)
2. Wait a sec, then HOLD "R" and power back on
3. You should now be in recovery mode, go to "Configuration"
4. Select "Game folder homebrew (currently x.xx kernel)" and change it so that it says 1.50
6. Exit - your PSP should now boot up as normal
7. Try running fastloader again
If it works, you probably had fastloader in the "GAME" folder instead of the "GAME150" folder.
P.s. you also need to check that fastloader outputs the ISO file to the ISO folder on the root of your memory stick and NOT FastLoader/ISO or UMDEmulator/ISO/.
Hope this helps??
if it still doesn't work - i'd suggest that maybe your fastloader files are corrupt? (try it on another PSP if possible).
But as i said earlier - you're better off using the new USB tool i mentioned