I recieve the next error after i attempt to validate a message page on my small site:

document type doesn't allow element "script" here

The element named above was discovered inside a context where it's not permitted. This would mean that you've improperly nested elements -- like a "style" aspect in the "body" section rather than inside "mind" -- or two elements that overlap (which isn't permitted).
One common cause of this error is using XHTML syntax in HTML documents. Because of HTML's rules of unconditionally closed elements, this error can make cascading down effects. For example, using XHTML's "self-closing" tags for "meta" and "link" within the "mind" portion of a HTML document could cause the parser to infer the finish from the "mind" section and the start of the "body" section (where "link" and "meta" aren't permitted hence the reported error).

I'm using Joomla 1.5.7, and also the doctype is

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Any idea how you can fix this?

First, you have to improve your Joomla install. 1.5.7 is almost 24 months old and you will find a large number of known security issues which have been patched. The present version is 1.5.22, you will get the patch package here - 1.5.7 to 1.5.22 patch

Next, the easiest way to many likely fix your validation problem is by using the modified version from the contact page that accompany the Beez template within the core install. Should you try looking in joomla/templates/bees/html/ you will notice a folder known as com_contact. Copy that to joomla/templates/[your template]/html/ which will in all probability repair the problem. If you've still got the issue then you will have to find the precise problem and what module/wordpress plugin is adding the code towards the page.