W7 Theme is a free script that change the look and feel of your Ubuntu desktop to resemble that of Windows 7. You can download either a GNOME and KDE version. The developer also offers links to download Windows 7 theme for Firefox to get as close as possible to the Windows 7 look.
Pretty convincing impersonation.
The only downside of this script is that there's no easy uninstall procedure at the moment. So be sure to make a backup before you install it in case you decide you want to switch back.
Also see the Windows 7 transformation pack for Linux.