Fix error

Because troubleshooting system problems is a complex process, and hardware and software configurations can vary greatly, there are many approaches you can take to fixing problems that might prevent Windows from starting (booting) correctly.

Try System Restore

Step 1: Start -> All Programs -> Accessories -> System Tools
Step 2: Click on System Restore

Determine if the problem is hardware-related

Step 1: Start -> Control Panel -> System and Maintenance -> Administrative Tools
Step 2: Click on Memory Diagnostics Tool

Options to try if Windows starts but runs erratically or slowly

System Restore
System Restore can return your system files to a point in time before you noticed a problem. System Restore works without affecting documents, photos, or other personal files.
Step 1: Start -> All Programs -> Accessories -> System Tools
Step 2: Click on System Restore.

Safe mode
You can use safe mode to temporarily disable programs that start themselves when Windows starts. If one of these programs is causing a problem, you can start your computer in safe mode and then uninstall the program before it starts.
You can access the menu by turning on your computer and pressing the F8 key before Windows starts

Options to try if Windows does not start at all

Startup Repair
Startup Repair is a Windows recovery tool that can fix certain problems, such as missing or damaged system files that might prevent Windows from starting. Startup Repair is located on the System Recovery Options menu, which is on the Windows Vista installation disc. If your computer has preinstalled recovery options, it might also be stored on your computer’s hard disk.

Reinstall
If no other options have solved the problem, and you are sure that your computer’s hardware is functioning normally, you might need to reinstall Windows. A custom (clean) installation of Windows will permanently delete all of the files on your computer and reinstall Windows, so only use this option if all other recovery options have been unsuccessful. After the installation, you will need to reinstall your programs and restore your files from backup copies.