If u dual boot (multiboot) between Vista and WinXP, or another OS, u can choose which one will start when turning on ur PC.
1. Open Computer
2. Click System Properties tab, and then click Advanced System Settings.
3. Click the Advanced tab, and then, under Startup and Recovery, click Settings.
4. Under System startup, in the Default operating system list, click the operating system that u want to use when turning on or restarting ur PC.
5. Check Select the Time to display list of operating systems box, and select the number of seconds u want the list of operating systems to be displayed before the default OS starts. Default is 30 seconds, but that could be an etternity for some PC users.
Happy compuying!