I've got MS Expression Web (12.0.6211.1000).
I usually use Homesite but it and Vista don't get on so I thought I'd try Expression for editing simple HTML files.
I find that Espression Web inserts the characters

at the top of every file I edit, just before the <!DOCTYPE...
tag. What are these and how can I make it stop!
That is the Unicode Byte Order Mark interpreted as something that isn't a Unicode character encoding (such as ISO-8859-1 AKA Latin-1).
The W3C has a guide to specifying which character encoding you are actually using.
If you don't want to use a Unicode encoding (I suggest you do, they are current standard rather than the ISO-8859 series which are legacy) then the forums suggest that you can change the encoding by going to (oddly) File > Properties... > Language and looking for a "Save the document as" list.