Windows 7 comes with Segoe UI Light version 5.00/0.90. Windows 8 (as of the Windows Consumer Preview) comes with version 5.15, which has been hinted at various sizes. This comes as a very welcome change, especially with Microsoft using Segoe UI Light all over the place in its latest webpage designs. I recommend grabbing the updated versions of Segoe UI from Windows 8 for your Windows 7 or Windows Vista machines.
As others have complained, the lack of hinting in version 5.00 is particularly noticeable with lowercase bs, ds, ps and qs, the ‘round parts’ of which don’t line up well with other characters. The heights of numerals are also variable at some sizes.
Have a look at the following screenshots for a better idea of the differences (click the images to see the full sizes).