What are the differences between Minesweeper for Windows XP and for Windows 7?

Beyond the innocuous cosmetic changes that you mention, it's not known (nor can I find any proven studies) whether the algorithm was changed as well.

However, it is known that the pre-Vista/7 versions of Minesweeper did not always start your first click on an empty or 0 square (that is, a square with no adjacent mines), while the new version always does. In both versions, though, you never lose on the first click.

This means in previous versions, your first click often opened only a single number, rather than a large, enclosed area. This made gameplay substantially more difficult as you had to continue clicking around to avoid guessing situations. However, in the new version, you always start with a 0 square, opening a minimum of 9 squares on your first click (the one you click plus the 8 squares surrounding it, which may or may not have numbers). This gives you a better head start for the most part, as you can easily find mines to flag right away.

Another difference is with chording: when you surround a numbered square with flags, you can chord by clicking on the number with both the left and right mouse buttons to immediately open all the unflagged squares around it. This may or may not set off mines, depending on whether you places the flags on the right squares or not. In the new version, you can also chord with a double left click on the number. When you attempt to chord with the wrong number of flags around a number, nothing happens in the previous versions, but in the new version the number will flash a red X signaling the error. You will still lose if you have the right number of flags in the wrong positions, though.

The new version also allows you to save your progress (like all other built-in games), as well as to restart a lost game with the exact same layout.

These are the main gameplay differences. Other differences include Freecell-like player statistics, tooltips, annoying dialog boxes and so on.

Minesweeper as a whole remains largely unchanged from Windows Vista to Windows 7.

I think there are only some UI changes in the versions for Windows Vista and Windows 7. The following blog entry gives nice details:

The UI design minefield - er... flower field??