Hi Eduardo,
please download and run the System Update Readiness Tool for Windows 7 (KB947821) [April 2010] and check if you can install the update.
After you run the tool a file called checkSUR.log is created in the folder C:\Windows\Logs\CBS. Please open it and look if the tool was able to fix all errors.
André
"A programmer is just a tool which converts caffeine into code" CLIP- Stellvertreter http://www.winvistaside.de/