Before upgrading the to the December 2007 Team Foundation Server Power Tools release, you should uninstall the previous version.  You can do this through Control Panel.   However, if you have uninstalled Visual Studio 2005 or upgraded to Visual Studio 2008 the TFS Power Tool uninstall may fail with a dialog box stating:

"Visual Studio Team Explorer must be installed on your system before installing Team Foundation Server Power Tools"

This is due to a bug where setup is looking for VS2005, something we've fixed moving forward.

That being the case, you will need to manually remove the existing Power Tools following these four steps:

 1.  Remove the Power Tools entry in the Add/Remove Programs tool.   Use this MSDN article and remove the entry where the DisplayName = "Microsoft Team Foundation Server Power Tools"

2. Use RegEdit to search for "Microsoft Team Foundation Server Power Tool" until you find the registry entry in:

HKEY_USERS\...user GUID...\Software\Microsoft\Installer\Products\...key...  where Product Name = "Microsoft Team Foundation Server Power Tools"

Use the same process to export the registry key (for backup purposes) as listed in the MSDN article and then delete this key.

3. From the Start->All Programs menu, right click on the "Microsoft Team Foundation Server Power Tools" entry and delete this from the Start menu.

4. Using Explorer locate and delete the directory under Program Files called "Microsoft Team Foundation Server Power Tools".

You should be able to cleanly download and install the December 2007 Team Foundation Server Power Tools now.