Just re-install WinXP on its previous partition. To reinstall the just erased Vista Boot manager, simply use the "Repair Your Computer" option from WinRE located on your Vista DVD.
QUOTE
Startup Repair is located on the System Recovery Options menu, which is on the Windows installation disc. Startup Repair might also be installed on your hard disk if your computer has preinstalled recovery options. Startup Repair might prompt you to make choices as it tries to fix the problem, and if necessary, it might restart your computer as it makes repairs.
If you have a Windows installation disc:
1. Insert the installation disc. If your computer is not configured to start from a CD or DVD, check the information that came with your computer.
2. Restart your computer. Click the Start button , click the arrow next to the Lock button , and then click Restart.
3. Choose your language settings, and then click Next.
4. Click Repair your computer.
5. Select the operating system you want to repair, and then click Next.
6. On the System Recovery Options menu, click Startup Repair.
http://windowshelp.microsoft.com/Windows/e...2f3f351033.mspxQUOTE
Windows RE is a combination runtime environment, diagnostic tool and repair system that basically tries to do about 80% of what one of the engineers in the Windows support team could do if they came to your home or office and hooked up a debugger up to your PC -- all without you having to do anything.
In looking at the support data, we know that the top 5 issues that cause Windows XP PCs not to boot are: 1) registry corruptions, 2) corrupt file systems (also known as NTFS metadata corruptions), 3) missing OS loader, 4) inaccessible boot devices (often caused by installing a bad storage driver) and 5) system file corruptions (some part of the OS getting deleted). These problems could happen because of hardware memory corruptions, disk corruptions, other hardware issues, buggy device drivers, or a kernel software issue. Regardless of how the system got into that state, the idea was to create an environment that would use heuristics to essentially implement a differential diagnosis to identify the issue and then use the resources in the other parts of the system configuration, combined with back-up data (such as system restore points) and a copy of key system image information to put the system back into an operating state -- without user intervention.
http://windowsvistablog.com/blogs/windowsv...nvironment.aspx