Untrue. In fact, you can install a fresh copy of xp and keep you old file structure in tact. I just recently upgraded my mobo, cpu, ram, vcard. I had a full post to my unregistered XP which I had to reload anyway. I reinstalled a new version of XP using the "update" method and all was well.
The OS has nothing to do with posting. So it definitely is not that. Connect the speaker and find out what the beep code is.
Report back.