Installed and tested VS '10 Pro with VM etc. Worked great, could open existing sketch save, etc.
A few days later went back to continue work and suddenly cannot open sketch, cannot start new sketch and now in twilight zone. 

Here is error if I try to open a new sketch
---------------------------
Visual.Micro.Visual.Studio.Arduino.10
---------------------------
Unable to create new sketch

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Here is error when attempt to open sketch.ino that worked before.
---------------------------
Visual Micro
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OpenOrCreateSketch: Unspecified error (Exception from HRESULT: 0x80004005 (E_FAIL))

Any help would be great and if you need any files etc let me know.

Thanks for the quick reply. No changes made to install location or libraries, BUT, I did install VS2012 as a result of 'special offer'. I believe this was the only change........

1. Restarted computer, launched VS, created Ard Project; same error.
2. Enabled Trace; no *.txt file created in install folder (Assembly Location).
Here is text from Output pane.

ExecCommand: Visual.Micro.Visual.Studio.Arduino.Helper.SolutionOpenArduinoSketch
ExecCommand: Visual.Micro.Visual.Studio.Arduino.Helper.SolutionNewArduinoSketch
*RefreshIntellisense      System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
   at System.ThrowHelper.ThrowArgumentOutOfRangeException()
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at processing.app.SketchEditor.get_SelectedBoard()
   at processing.app.SketchEditor.getBoardPreferences()
   at processing.app.SketchBuilder.get_BuildFolder()
   at processing.app.SketchBuilder.build(Boolean verbose)
   at Visual.Micro.Visual.Studio.Arduino.AddInApp.RefreshIntellisenseOfSketch(Project oProject, SketchBuilder sk)

Thanks,

Question

In tools>options>Visual Micro>User Interface. Please switch on Trace and then reset the addin and send me the contents of the trace window that appears after vs has started.

Thanks

I think I found the Micro System Trace pulldown, but it is not always there.
It only shows up if I first launch an existing Arduino project that then fails as noted in first message of Unable to create sketch etc.
 
And then it shows up the first time I reset addin (this is probably what you meant). Here is the message:

CreateCommandBarItemsPerm      Begin AddinVer=27
Invalid Visual Studio SETUP Request Detected      Exiting visual micro setup due to an invalid visual studio cm_uisetup request!
CreateCommandBarItemsPerm      End

good news thanks for the update

the best thing to do is to sign up for the microsoft websitespark program which gives you a 3 year license for vs 2010.

2012 isn't tested yet. it will probably fail but if you want to test its quite easy.

1) open the [vm.10].addin xml file in the vm program folder and you will see that it states Visual Studio 2010 as valid option(s). You can copy them and make a 2012 set.

2) in 2010 under options>macro security>search paths you will see how a path to vm was added by the installer. add the same path to 2012 options>macro security>search paths

Those are the two options that enable vm in 2010 so in theory should be the same in 2012. However when I told you about the 2012 free download I was passing on a tweet from the vs team. They failed to point out that the "free" offer liencense expires in January 2013. Doh!

So the websitespark program (see link on visualmicro.com home page) for 2010 is the best option.

Pls let me know what you do and how you get on