The main problem with your RSS feed is that you have a blank line at the start of it. There's also a bunch of crap at the bottom, but since that's your host adding that crap, there's likely little you can do about it.
To get rid of the blank line in your feed, look for a file you may have modified that has a blank line either at the very beginning or very end of the file. It's probably a plugin you're using. Remove the blank line. That will solve most of your problem there.
With regard to your XHTML errors:
- META is not valid. All XHTML tags are *lowercase*. Change it to meta. Same for any other uppercase tags you find.
- That Amazon iframe: Remove the border="0" from it. border is not a valid attribute of an iframe. Yes, that's Amazon's fault, I've had it too.
- The form at the bottom of the page is inside one big paragraph tag. That's not valid. You can't put forms inside paragraphs. Same goes for the tables and such. Rework this whole section, would be my advice.
- Whenever you have an attribute for an element, like <input type=radio...>, you must put the attribute value in quotes. Always. These are not optional. That should be type="radio" there. Same goes for the rest.
- CHECKED is not valid for a radio button. First off, it's all caps. Second off, it should be checked="checked" to be valid XHTML. In XHTML, all attributes must have values. Same goes for those <td nowrap>'s. Make them <td nowrap="nowrap">'s insetad.
- Input tags (radio buttons anyway) don't have content. It's not like <input>stuff</input>. It's <input />stuff. These input tags should be empty.
- A textarea must have both rows and cols attributes. They are not optional.
- All tags must be closed. That includes
<br>. Make it
<br /> instead.
- Finally, that crap at the end of the page... get rid of it, if at all possible.