Zorin OS is an Ubuntu based linux alternative for windows users. The look and feel is that of Windows 7, and yes you can change it to the feel of Windows XP, Windows Vista, Windows 2000, Linux (GNOME 2.x) or Mac OS X. Anyone wishing to try out Linux and simply have no time to go through the steep learning curve, Zorin OS alleviates much of that pain. The Core, Lite and Educational versions are available to download for free from the Free download page. The Premium versions (Business, Gaming, Multimedia and Ultimate) are available in exchange for a donation on the Premium page for a physical DVD or a download.

If you have an older system, then Zorin OS is worth giving a try.
System Requirements

For the GNOME based versions (non Lite):
700 MHz x86 processor
3GB of Hard Drive space
376 MB of system memory (RAM)
Graphics card capable of 640×480 resolution
Sound card

For the LXDE based versions (Lite):
266 MHz x86 processor
2GB of Hard Drive space
128 MB of system memory (RAM)
Graphics card capable of 640×480 resolution
Sound card