If you have XP then:
Step 1: Determine whether there is a Windows.OLD folder and whether there is sufficient free space on the Windows hard disk
1. Click Start
Collapse this imageExpand this image
, and then click Computer.
2. On the View menu, click Details.
3. In the Free Space column, note how much space is available for Local Disk (C under the Hard Disk Drives area.
4. In the Hard Disk Drives area, double-click Local Disk (C:), and then determine whether the Windows.OLD folder exists.
Important If the Windows.OLD folder does not exist, you cannot follow the steps in this article to restore the previous Windows installation to this computer.
5. Right-click the Windows.OLD folder.
6. Windows Vista will determine the size of the folder after several seconds.
Determine whether the Windows.OLD folder is smaller than the free space that is available for Local Disk (C in step 2.
Note If the Windows.OLD folder is two times as large as the free space that is available for the Local Disk (C entry, you may be unable to restore the previous Windows installation.
Step 2: Start the Windows Recovery Environment
1. Put the Windows Vista installation disc in the DVD drive, and then restart the computer.
2. Press a key when you are prompted to restart from the disc.
3. In the Install Windows window, select a language, a time, a currency, a keyboard input method or other input method, and then click Next.
4. In the Install Windows window, click Repair your computer.
5. In the System Recovery Options window, click the version of the Windows Vista operating system that you want to repair, and then click Next.
6. In the System Recovery Options window, click Command Prompt.
The Command Prompt window opens, and it displays the command prompt. The command prompt is where you will type the commands that are described in the following steps.
Next steps: http://support.microsoft.com/?kbid=933168&SD=tech