If you have done something wrong with your computer , one of the best things you can do is perform a system restore where you can take your computer back in time to a date when everything was working properly.
System restore can be accessed in 2 ways.
From Safe Mode and from System Tools , Accessories
Restart your computer
During the boot process, press F8 to enter Safe Mode
Select Safe Mode with Command Prompt
Select the OS (if multiple boot options are shown)
Select your User Account
A command prompt window will open
For Windows 7 type rstrui.exe and press Enter
For Windows XP type %systemroot%system32restorerstrui.exe
System Restore will open
Select Restore my computer to an earlier time
Click the Next button
Select a date you would like to restore your computer to and click next
Windows will inform you when the process is complete