You tried booting without a HD? What did you expect to happen? Just sayin....
1) What is new or different since the last time everything worked properly (ie, new hw, new sw, virus, error, etc)?
2) Go to the website of your hd manufacturer and download their diagnostic. Burn it to cd, boot to it, run it.
3) To check the ram, download memtest (http://memtest.org/
). Burn it to a cd using a dedicated .iso burning utility (http://www.petri.co.il/how_to_write_iso_files_to_cd.htm
), make sure the cd drive is at the top of the boot order in bios, then boot to the newly created cd and run the utility.