I'd go with what pete said, make sure excel is fully updated.
First though I would add instructions to fully update the OS and .net 3.5. V4 doesn't seem to like ninja...not the problem though, NT won't even start with V4 standalone, but corruption, or filesystem changes with V4 installed after 3.5 may result.
Maybe add links for the full .net versions instead of the painfully slow bootstrapper
After that, other things to try are:
control panel > add remove programs > find .NET framework 3.5 SP1 in list > click change / remove > select repair option.
Still no joy after that, uninstall NT .NET and excel.
Reinstall .NET NT and excel from scratch.
NT does store and reference files in the .NET architecture - so the above eliminates
any corruption from other programs that have modified .NET
Really a dedicated rig or at least a dual boot is best for any critical work to avoid this kind of crap in the first place
Full .NET link:
http://download.microsoft.com/download/2/0/e/20e90413-712f-438c-988e-fdaa79a8ac3d/dotnetfx35.exe