I've read the ENTIRE thread and can't believe someone would suggest just CHANGING the PRAM battery. Computer users are apt to consider this little piece to be eternal. That is not the case. Typically, a PRAM battery lasts up to 10 years in an iMac (depending, of course, on daily usage). But when this battery is DEAD, your Mac just refuses to start (I don't explain why here, but it should be straightforward to understand). If everything is working properly (as reported by guido), then the culprit is probably... the PRAM battery.