Often, if you copy and paste a Flash object into your own code, it won’t validate as XHTML because HTML tags or invalid tags are used. One way of solving this is the Flash Satay method, which is explained far better than I can explain it by Drew McClellan.
I used this for the Last.fm widget, which was subsequently removed, and the embedded youtube video in one of my blog entries, although I didn’t need to use the whole hog. Simply removing the invalid tags and properties and adding the correct properties in the correct place worked for me because the widget wasn’t too horrendous, and the style section wasn’t even needed. I didn’t bother with the workaround because if you (voluntarily) use IE, especially older versions of it, then you deserve all you get. Apologies to those who are forced to use it; but perhaps you should have a word with your system administrator!