I have to two hard disk in my computer. first HDD has drive C D and E, while second HDD has only drive F. Windows xp was installed on my first HDD(in drive C). Last month I installed windows vista on my second HDD(i.e. in drive F). Whenever I started my computer it asked me to select the OS from both of them. Recently I have reinstalled windows xp(on the same HDD and same drive)because of a virus attack. But after reinstalling the windows xp my computer does not ask me to select the OS. OS selection option does not appear and windows xp starts directly. I am unable to run windows vista. What is the solution of it?

I have to two hard disk in my computer. first HDD has drive C D and E, while second HDD has only drive F. Windows xp was installed on my first HDD(in drive C). Last month I installed windows vista on my second HDD(i.e. in drive F). Whenever I started my computer it asked me to select the OS from both of them. Recently I have reinstalled windows xp(on the same HDD and same drive)because of a virus attack. But after reinstalling the windows xp my computer does not ask me to select the OS. OS selection option does not appear and windows xp starts directly. I am unable to run windows vista. What is the solution of it?

Read this link on how to repair your problem

How to dual boot Vista and XP (with Vista installed first)

Ther reason why your system no longer shows Vista is because the bootloader of Vista has been overwritten. The bootloader is always stored on the first hard drive's Master Boot Record and the first partition of the first hard drive - both of which were overwritten when you reinstalled XP.

In order to correct this problem, you will need to download and install Vista Boot Pro. This tool can be installed under either (or both) of your operating systems and is designed for managing the Vista boot system.

In VBP, yo will want to first click on the Bootloader button and then select the option to install/reinstall the Vista Bootloader. After this is complete, click on Manage OS Entries. Here you will need to Add a New Operating System Entry for each Windows XP and Windows Vista. After creating both entries, be certain to select the operating system you want to boot to normaly and check the box titled Set as Default. Once these changes are complete, apply the settings.

You should now be able to reboot your computer and be able to select which OS you wish to use. Make certain you can boot into both OS's properly.

After the repair is done, boot into Vista, install Vista Boot Pro, and then use the Backup/Restore Center option to create a backup of your boot configuration - so you will have it if you ever need to go through this again.