First, Security Essentials gives you detailed information what it found and where it was found. Please provide these details, you can review these from Security Essentials' history.
Second, please be aware that Diaspora was packaged in one humongous executable file (which was a stupid thing to do). Every single on-access anti-virus scanner will attempt to scan it thoroughly as soon as you open folder containing the file. This may result in Windows Explorer freezing, locking up or in extreme cases, even crashing (depending what on user does in confusion) until the scan is finished. Depending on anti-virus software in use as well as performance of your PC, this may take a good while.
My suspicion is that this "trojan" was found somewhere else, likely where the web browser you use stores temporary files. Or like mentioned earlier, was false positive. The explorer freezing, locking up or crashing is explained by on-access scanner doing its "magic" on the humongous Diaspora installer executable. Normally Explorer won't crash, but when it freezes and user attempts to operate it, Windows says it's not responding and user tries to close it, thus resulting in "crash".