These instructions are for Windows 7. Show Windows Vista instructions
Because Adobe does not provide a 64-bit plugin for their Flash player, gadgets that require Flash (such as Pandora) will not operate on 64-bit Windows. To get around this problem, you can run the 32-bit version of the program that manages the gadgets. Here's how:
This is the easiest way, and the one I recommend. Adobe has recently begun offering a beta Flash player for 64-bit browsers. To get it, visit its download page and download the "active-x plugin for 64-bit Windows — for Internet Explorer only". After installing it, right click the Desktop, go to view, and uncheck "Show desktop gadgets". Then, do it again to restart them.
64-bit Windows comes with a 32-bit version of sidebar.exe, the program that manages the gadgets. You can set this one to run on startup instead of the default 64-bit one.
The only problem I've noticed with this so far is that Windows 7's weather gadget won't run in 32-bit mode for some reason.