Many of us need to use more than one operating system from time to time. Perhaps you like to use Linux for day-to-day tasks while leaving Windows XP in place to run games, or you might want to check out the new features in the Windows 7 beta without damaging an existing copy of Vista.

Often the simplest way to run a second operating system is to use virtualisation software, such as the free Virtualbox or Microsoft’s Virtual PC. For demanding tasks, though, this method just isn’t fast enough. What you need is to dual boot: to install both and choose which to use when the PC starts up.

This can be hard to set up, though, and if you make a mistake there’s always a chance that the computer will refuse to run anything at all. In this back to basics article we’ll explain how to set up and troubleshoot dual-boot systems.

Adding Vista to XP
One of the most common dual-boot setups is to run Windows XP and Vista alongside one another on one disk. This can be very simple to set up, but the method depends on what you’re starting with.

If you’re starting with a Windows XP computer and adding Vista, the process of dual booting is simple. If there’s room on the disk for a new primary partition then add one, or if not make room first using a tool such as Sourceforge's Gparted.

If you’re unclear about the difference between primary and logical partitions, see 'Partition types' at the end of this article. Install Vista into the new partition and it will automatically create a boot menu with two options: ‘Earlier version of Windows’ and ‘Microsoft Windows Vista’.

By default the menu will wait 30 seconds and, if nothing is chosen during that time, start Vista. You can change both the time limit and the default, however.

In Vista, start a command prompt with administrator privileges: press Start, type cmd, right-click the cmd.exe program shortcut that appears and choose Run as administrator. To alter the time limit to 5 seconds type:
bcdedit /timeout 5

To change the default option you need to know what each is called. Type bcdedit /enum and a list will appear. Look for the paragraph that lists the XP boot settings and find the identifier: this will normally be {ntldr}. Use the bcdedit command to make this the default: bcdedit /default {ntldr}

Adding XP to Vista
The only real problems with this setup appear if you install XP after Vista rather than the other way around. This is because, whereas Vista was designed to look out for an XP installation and accommodate it, XP’s setup program will simply ignore Vista and set itself up as the only option. In order to recover from this you’ll need a Vista installation DVD.

If you find that after installing Windows XP your copy of Vista is inaccessible, restart the PC and boot from your Vista installation disc. Choose English UK at the first screen and at the second click the ‘repair your computer’ link.

The setup program may find the problem and offer to repair it automatically: if so, choose ‘Repair and restart’. If not, click Next at the list of operating systems and choose to open a command prompt. Next, type:

bootrec.exe /fixmbr
bootrec.exe /fixboot

These commands will remove XP from the hard disk’s Master Boot Record (MBR) and reinstate the Vista loader. When you restart the computer Vista should become available.

Sometimes after doing this Windows XP will disappear from the boot options and Vista will start immediately. If this happens you could boot Vista and use the bcdedit command to reinstate a Windows XP entry in the boot menu.

For a less fiddly approach, though, download the EasyBCD tool. This is a free graphical interface for bcdedit that makes the process far quicker.

Once installed, run EasyBCD. Click Add/Remove Entries from the menu, select the Windows tab and set the Type dropdown to ‘Windows 2K/NT/XP/2K3’. Give the entry a name, then click the Add Entry button and then Save. On restarting the PC you should see a new option for Windows XP.