Even though you've figured it out, I'm going to reply anyway for the sake of those who are stuck in this (oh so common) predicament.
It does depends on how you've structured you html, but this is how I would do it.
HTML
Code:
<form action="xyz" method="post">
<label for="searchSite">Search</label>
<p>
<input type="text" name="search" id="f_searchSite" class="search-field" />
<input type="image" src="path/to/image.gif" class="submit" />
</p>
</div>
CSS
Code:
input.search-field { float: left; border: 0; background: #fff; width: 100px; }
input.submit { float: left; width: 25px; height: 25px; margin: 0 0 0 -25px; }
now lets break this down.
its a common issue where by the submit button and the input field don't line up correctly when you've got them side by side. the best and easiest method is to float them both (to the left in this case) to line them up.
working off the OPs image, the submit button is actually over the top of the input, so thats where we apply the negative margin on the left of (in this instance) the width of the submit button.
personally, if I was building a similar search field, my submit button would actually be a transparent gif with a matt white edge so that it will always look 'inside' the input field behind.
very quick application to fixing the uneven search options. and this method is definitely open to variations.
hope this helped.