Microsoft Expression Web 4 won't open

I saw older messages about this on the old forum, but all of them had different error messages than the one I receive. Expression Web 4 was working fine -- no new software added, nothing unusual going on, but then, two days ago when I tried to open Expression Web 4, a message appeared saying there was a problem and it couldn't open. I clicked for the app to look for a solution online, but nothing came back. I downloaded this from the Internet when Microsoft was offering a free upgrade to 4 for 3 users, so I don't have a CD to reload. I clicked to see the details of the problem and here's what it said.  What can I do?

Problem signature:

Problem Event Name: CLR20r3

Problem Signature 01: expressionweb.exe

Problem Signature 02: 4.0.1303.0

Problem Signature 03: 4e117236

Problem Signature 04: System

Problem Signature 05: 4.0.0.0

Problem Signature 06: 4ee9b7da

Problem Signature 07: 3c58

Problem Signature 08: 11f

Problem Signature 09: System.Security.Security

OS Version: 6.1.7601.2.1.0.768.3

Locale ID: 1033

Additional Information 1: 0a9e

Additional Information 2: 0a9e372d3b4ad19135b953a78882e789

Additional Information 3: 0a9e

Additional Information 4: 0a9e372d3b4ad19135b953a78882e789

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