Když pustim PC mam černou obrazovku

Most likely a damaged or altered registry entry. Clicks start and type
'regedit' and hit <enter>. Expand the keys at left to reach:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Click on this key, then locate the shell string in the right pane. Double
click it, replace the contents with just: explorer.exe
Click ok, then close the registry editor and reboot.

http://social.technet.microsoft.com/Forums/en-US/i tprovistadesktopui/thread/1a82be8e-eaf1-4036-8f5b- eaa062e59c50

Snad se nejdřív podívej na registry.