vs2010 suddenly won't compile

Hi, using vs2010 Pro on an XP sp3 machine.  Suddenly none of my projects compile !, this began with an error string "Unable to access", however I now note that even a straight C++ template will not compile, the only error is "failed to compile".  I know I need to pass on more info, but I'm not sure how to get it as I'm new to vs2010.  A day ago, all of my projects compiled OK, no changes have been made, no new installations, except updates, SP1 for vs2010 installed.

So my first question, Where do I start to find out what's wrong ?

Thanks, Mark