Yeah and when asking for help why not expand a little on what you want? Posting a picture of a Windows wallpaper and then asking to "switch the color of the theme" is pretty vague.
If, like everyone else has been leading me to believe, you just want to change the hue then you can do what people said above, but there is a better way.
Open up the image in Photoshop and add a Hue/Saturation adjustment layer. In Photoshop CS2, the icon to do this is at the bottom of the Layers palette, fourth one over. Clicking it will bring up your available options, just choose what I said above. An alternate way to do it is to go to Layer » New Adjustment Layer » Hue/Saturation...
Once you have selected it, a dialog will come up with a few sliders. If you look down at the bottom there's a Colorize check box -- check that -- and now your image should become monochromatic (one color). Doing this will have the same effect as creating a new layer above the image, filling it with a color, then setting the blending option to Color. But doing it this way gives you a lot more control.
To change the color, move the Hue slider left and right to the desired hue, if it's not vivid enough, then move the Saturation slider, etc.
But doing this, like other people said, will affect the Windows logo as well. You'll need to separate it from the rest of the image if you want to keep it the same colors.
Edited by sq3r, 17 January 2006 - 12:26 AM.