How to Reset Your Computer Without a CD
Resetting or repairing your computer can be necessary when system problems have rendered your PC unstable or error-prone. This can be done through the use of the System Restore utility in your computer's operating system, which will enable you to turn back time as far as is necessary to reverse your computer issues, even back to the day your operating system was first installed.
Click "Start," open "All Programs," open "Accessories," and open "System Tools."
Open "System Restore."
Click "Restore this Computer" for XP or select "Choose a different restore point" for Vista and 7.
Select the most distant restore point that you can access. This will reset your PC to its default condition.
Press "Next" to begin the automated system restore procedure.