Use of "an" and "a" preceding a noun

What is the correct rule to determine use of "an" or "a" preceeding a noun or proper noun, please?

For example "A IBM computer" or "An IBM computer"?

The latter "feels" as if it is correct, but because of examples like "An hotel" which are correct, but don't feel so, this subject has caused a lot of debate with my friends! The truth is, no one can actually recall a hard and fast rule.

Many thanks.

Well, I just found part of my answer:
http://www.usingenglish.com/glossary...e-article.html

I guess therefore that "An IBM computer" is correct, even though without the "IBM" we would sat "A computer"?

This still doesn't explain "an hotel", and other exceptions, though!
May be we should speak with a French accent - "An otel"