Ok every developer turns off UAC right :) so when you come to install the Live Mesh desktop client you are prompted with the following

If you look through the Live Mesh teams blog you come across this post which states the following reason why

Why does Live Mesh require UAC?

At this point it’s worth talking about UAC and why the Live Mesh client requires that UAC be turned on. If you’ve tried to install Live Mesh on a system with UAC disabled you’ll have found that the installer blocks on this configuration.  There’s a technical reason why we impose this restriction:  In Vista RTM with UAC off, COM does not read the per-user hive, meaning COM objects registered per-user do not work.  Since Live Mesh installs per-user without elevation it is subject to this restriction.  To prevent users from installing on unsupported configurations we added a block in our installer when UAC is off.  Fortunately there’s light at the end of the tunnel for users that run with UAC disabled.  Windows Vista SP1 has a change that allows per-user COM when UAC is disabled.  With the release of SP1, we have begun validating that Live Mesh fully functions with UAC disabled, but haven't yet finished our testing.  Expect this configuration to be supported in a subsequent refresh.   As you can probably guess, this also means that even after you install, UAC must remain enabled for Live Mesh to work correctly.  If you turn UAC off, we won’t be able to find any of our COM objects.

So technically you can do it on Vista SP1 but it's most likely not going to behave like you expect. I'd just turn UAC back on and see how it goes, UAC should be less annoying on Vista SP1 if not then it's going straight back to the cave it crawled out of.

Ok now how do you turn UAC on/off, well you did turn it off surely you know how to get it back on :) but alas you forgot after 18 months of blissful non UAC annoyance.

Method 1 - Using Control Panel

1. Click Start and then open Control Panel.
2. In the Control Panel, click User Accounts and Family Safety.
3. Click User Accounts.

***** STOP RIGHT HERE *****

Ok did you just do the above 3 steps? If so then why didn't you just go Start and type "User Accounts" into the search box and open from there? You do have Vista right that's why you are turning it off. Geez some habits are just hard to break. 

***** CONTINUE *****

4. Click Turn User Account Control on or off.
5. Now check the little box with disclaimer about giving up your first born child (just checking if your still reading or busy REBOOTING from the change)



6. When prompted, restart the computer. Note that the changes will affect all users on the computer.
7. To disable the UAC, simply tick or select the checkbox again.

Method 2 - Using Registry Editor

1. Run Registry Editor by typing “regedit” in Start Search or command prompt.
2. In Registry Editor, navigate to the following registry key:

   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\
   CurrentVersion\Policies\System

3. Locate the following DWORD registry subkey in the right pane:

   EnableLUA

4. Right click and choose modify or double click on EnableLUA to modify the setting. On valud prompt, set the new value to 0.
5. Exit from Registry Editor.
6. Restart the computer.
7. To enable the UAC again, simply change back the value of EnableLUA to 1.

***** Did anyone REALLY just do the above? Come on you just hacked the registry for what reason?, does that make you feel like a l33t HaX0r? *****

Method 3 - Using MsConfig System Configuration

1. Run MsConfig from Run option.
2. In System Configuration window, click on the Tools tab.
3. Scroll down and locate "Disable UAC" option item. Click on that line and press "Launch". Why "Launch" and not "Run", "Execute" or just plain "Make it so"



4. A command prompt window will open and automatically execute and run certain process to disable UAC.
5. Close CMD window when done.
6. Close Msconfig.
7. Restart computer for changes to apply and effective.
8. To re-enable UAC, simply select "Enable UAC" instead of "Disable UAC", and then click on Launch (Make it so) button.

You also have option of Group policy and using 3rd party tools like TweakUI but hey by now you've already rebooted installed Mesh and had your first cappucino of the day.