Some more information would be helpful...

What currencies do you have installed?
What is your default currency?
What is your URL?
Does this happen with *all* orders, or just when you use a coupon?
What "should" the amount be?
What is your tax configuration on your site?
Are you using gift-certificates to do any partial-payments?
Are you hosted on WIndows or Linux? IIS or Apache?
What version of PHP are you using?
Do you have SSL enabled?