My guess is that either your download is corrupted or something else is causing the problem (video driver, background task, etc). If you look in the Windows Event Viewer you should be able to get more information about the crash (Start > Control Panel > Administrative Tools > Event Viewer).
What changed between the time it was working and now? If you extract whatever test patch you were using previously (U18?) does it work or crash? If U18 works, then it could be a corrupt download or a valid bug.