Pictures look different in Photoshop

Ok, this may be a complete dumb question, but why do my pictures (unedited jpegs) look less saturated in photoshop compared to the simple windows picture viewer. I know I don't have a calibrated monitor, but is there a setting somewhere in photoshop to get the colors to match better. Or is just windows does not display the true colors. I am trying to learn the ins and outs of photoshop and this was just one question that came about.

Thanks

Make sure your preference is set correctly in PS

My preference is matching PS, my RAW program, and printer. Not worring about any other program that might open the image makes things easier.

Windows Picture Viewer isn't colour managed, Photoshop is. In other words, Photoshop attempts to use whatever monitor profile you have configured to display the image accurately, whereas Windows Picture Viewer simply displays the image.

It is worth trying to calibrate your monitor - even if it's just loading the manufacturer's profile and running Adobe Gamma. It's easier to use a colorimeter based calibration setup, and the results are almost certainly superior, too.

David