Before you can get started installing Omnimo, you'll need to download and install the latest version of Rainmeter for your platform; if you're running on 64-bit Windows, you'll want to make sure to grab the right download. Rainmeter is an awesome Windows customization tool that we've featured plenty in the past, but the upshot is that Rainmeter provides the framework for the Omnimo Windows Phone 7-like customization; the Omnimo download and installation (which we'll detail below) provides the skins and layouts necessary to transform your desktop.
After you've downloaded it, you'll need to open Rainmeter at least once before proceeding to the Omnimo installation, because there's a number of first-time initialization routines that create the appropriate configuration files necessary to install Omnimo. Once you've finished installing Rainmeter (a simple installation wizard will walk you through it) and running it for the first time, you can exit Rainmeter via its system tray icon to proceed to the next step.
Now it's time to download and install Omnimo UI
If you don't already have the Segue UI Light font installed on your computer, you'll need to install the included font file via the right-click menu, or by dragging it into the Fonts folder. On my Windows 7 test machine, this font was already there, but it doesn't hurt to double-check, or just re-install it to make sure. Windows will warn you if it's already installed, so you can just stop the installation if you get that warning. (I'm guessing it's a font included by default in Windows 7, but it may not be available in your XP machine, for example.)
Now that you have all of that out of the way, you can launch the Rainstaller utility included in the Omnimo download; it will add the skins and themes to the appropriate Rainmeter folders. You'll want to make sure you have Rainmeter closed before running through this process.
When it fires up, click on the Custom Install button if you need to adjust where it installs the skins; the Express Install should work for most people (if you installed Rainmeter with all the defaults, for example, you should be fine choosing the Express Install). When the installation is done, keep the Launch Rainmeter on exit checkbox ticked and click Finish (well, there's a typo, so technically you're clicking Finnish).