Xml does not allow most special characters (such as &,;,%,etc...).
To use these in your markup, (although I would avoid putting querystrings in a sitemap file. Try using mod_rewrite if you are running apache, or http://www.isapirewrite.com/
if you're running windows) These will rewrite your urls into "my-page-name/" instead of "page.php?i=1&p=2".
Anyway, back to the markup.
For & symbols, use & wherever you need an amphersand and you should be fine. I would also kill any semi-colons you have in there just to be safe.
Here is a link to other codes for special characters:
Hope this helps.