You cannot select the Upgrade option when you try to install Windows Vista, and you receive a "Windows cannot determine if another operating system exists on your computer” error message

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Step 1: Save a backup copy of the Boot.ini file

1. Click Start, click Run, type sysdm.cpl, and then click OK.
2. In the System Properties dialog box, click the Settings button on the Advanced tab of the Startup and Recovery area.
3. In the System Startup area, click Edit. This step opens the file in Notepad for editing.
4. In Notepad, click File on the Menu bar, and then click Save As.
5. Right-click in an empty area of the Save As dialog box, point to New in the shortcut menu, and then click Folder.
6. Type a name for the new folder, and then press ENTER. For example, type temp, and then press ENTER to create a folder named temp.
7. Double-click the new folder named temp, and then click Save to save a backup copy of the Boot.ini file.

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Step 2: Edit the Boot.ini file

1. Click Start, click Run, type sysdm.cpl, and then click OK.
2. On the Advanced tab, click Settings in the Startup and Recovery area.
3. In the System Startup area, click Edit.
4. Find the line in the Boot.ini file that contains the name of the previous operating system. For example, you may find a line that resembles the following.

   multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect

   If the Windows Vista Setup program created a partition in the unallocated disk space in front of the partition where a previous operating system is installed, the previous operating system partition must be marked as partition 2. In this scenario, you must increment the partition value by one. For example, if the Boot.ini file contains "partition(1)" for Windows XP, modify this text to "partition(2)." The modified line resembles the following.

   multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /fastdetect

5. After you make the correction, click Save on the File menu to save the new version of the Boot.ini file, and then exit Notepad.

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