Does this happen in all browsers or just in certain browsers? If you are having issues in just IE, then most likely your page does not have a doctype declaration before the <html> tag, this will cause IE to render using quirks mode which is not CSS compliant.
make sure your page has a doctype at line 1:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
if you are still having problems, please provide a link.