Visual Studio crashes when typing in editor

I have been using VS2010 for a few weeks but for the past few days it has been totally unusable. When I type anything into the editor Visual Studio throws an exception, then I get lots of intellisence errors (the code is fine, I can compile it if I run before trying to use the editor) - here is the log message:

  <entry>
    <record>241</record>
    <time>2011/01/25 08:30:34.109</time>
    <type>Error</type>
    <source>Editor or Editor Extension</source>
    <description>System.InvalidCastException: Unable to cast COM object of type &apos;System.__ComObject&apos; to interface type &apos;Microsoft.VisualStudio.TextManager.Interop.IVsTextReplaceEvents&apos;. This operation failed because the QueryInterface call on the COM component for the interface with IID &apos;{CF9928D9-65AE-4319-A446-94ED5C45ECDE}&apos; failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).&#x000D;&#x000A;   at System.StubHelpers.StubHelpers.GetCOMIPFromRCW(Object objSrc, IntPtr pCPCMD, Boolean&amp; pfNeedsRelease)&#x000D;&#x000A;   at Microsoft.VisualStudio.TextManager.Interop.IVsTextReplaceEvents.OnReplace(ChangeInput[] pCI)&#x000D;&#x000A;   at Microsoft.VisualStudio.Editor.Implementation.VsTextBufferAdapter.OnTextBufferChangedHighPriority(Object sender, TextContentChangedEventArgs e)&#x000D;&#x000A;   at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.RaiseEvent[TArgs](Object sender, EventHandler`1 eventHandlers, TArgs args)</description>
  </entry>
  <entry>
    <record>242</record>
    <time>2011/01/25 08:30:34.125</time>
    <type>Error</type>
    <source>Editor or Editor Extension</source>
    <description>System.InvalidCastException: Unable to cast COM object of type &apos;System.__ComObject&apos; to int