别处看来的, 记录一下, 免得以后找不着了.

1. Run the .net cleanup utility from http://astebner.sts.winisp.net/Tools/dotnetfx_cleanup_tool.zip

2. Make sure there are no .Net versions from add/remove programs

3. renamed the C:\Windows\Asssembly folder and restart

4. On restart, install the .Net Framework 3.5 from the link http://download.microsoft.com/download/6/0/f/60fc5854-3cb8-4892-b6db-bd4f42510f28/dotnetfx35.exe