To use System Restore to restore Windows 7 to a previous state, follow these steps mentioned in the below link: http://windowshelp.microsoft.com/Windows/en-US/Help/517d3b8e-3379-46c1-b479-05b30d6fb3f01033.mspx#EI
For your information:
http://windows.microsoft.com/en-US/Windows7/User-accounts-frequently-asked-questions