You may be unable to log into Windows XP after installing Windows Vista and making your computer a dual OS / dual boot machine. This primarliy happens when Windows Vista is installed in unallocated space on the hard drive that existed in front of the Windows XP partition. To resolve this issue, follow the steps below:

(Please note that you will need administrative rights on your login to perform these steps.)

1. Boot the computer into Windows Vista
2. Right click on the “Start” button and on the pop up menu, click on “Explore“
3. Click on ”Organize” option and then on the drop down menu, click on “Folder and Search Options“
4. Click on the “View” tab
5. In the advanced settings area, click “Show hidden files and folders“. Click on the checkbox next to “Hide extension for known file types” to uncheck it and then click on “Hide protected sytem files (Recommended)” to uncheck it. Click “yes” to confirm that you want to display operating system protected files and then click on “OK”
6. Locate the Windows XP drive by using the “Computer” panel browsing (Start–>Computer)
7. Now locate “boot.ini” file. Right click on it and click on “properties” on the pop up menu
8. On the boot.ini properties window, click on Security tab and then click on “Advanced“
9. Click on the “Owner” tab and then click on “edit“. If you get a prompt to enter administrator password, type your password to continue to the next step.
10. In the change owner list, Click your alias or leave the default option selected. Now click on “OK” 4 times.
11. Now right click on boot.ini file again and click on “Edit” on the pop up menu
12. On this window, modify the Advanced RISC Computing (ARC) path to the correct Windows XP partition

These steps will solve your issues with Windows XP booting and allow you to enjoy both the operating systems compleltely.