It's unlikely you'll ever be able to run Windows 7 on a mobile phone, but at least you can pretend, with Seven Theme.
This theme transforms your Series 60 device, adding a swish, multi-colored background wallpaper based around the Windows logo. This background also appears behind the various menus within your Symbian device. Unfortunately, this causes some problems on certain menus, where the black text becomes unreadable due to the background color.
The Seven Theme also brings a few new icons to your system, and many of these are loosely based around what you'd find in Windows 7. Unfortunately, it doesn't amend all of your system icons, so the menus end up feeling a little disjointed.
If you're madly passionate about Windows then you may like the Seven Theme, but personally I found it distinctly average.