You can use software like Partion Magic or Windows Xp Installation CD; boot up in Recovery mode, try to repair MBR with the command fdisk /mbr after that restart the system and using the Xp disk in Recovery mode make partions for Xp.
Format the drive and install a new operating system in its place.
You cannot format the drive / partition that Windows Vista is currently running on while it is running. You will need to boot from either a partition utility disc, such as GParted, or the install disc of the operating system you want to install, and format the drive from there.
Yes. Actually, there is no need to remove the hard drive. You can format the existing hard drive and install XP on it as well. Note: Yes, you can there is no need to remove the hard drive. You can format the existing hard drive and install XP. and also you can install both opreting system single computer. XP and vista both.
There is no "DOS mode" for a hard drive, and it is not necessary to use DOS to format a hard drive. Vista's installer had a built-in partitioner and formatter.
Format and reinstall.
No special configuration or software is necessary; just plug them in. If the drive has not been formatted, you may need to format it (right-click the drive in "Computer" and select "Format").
The main drive is the drive with your operating system on it.
There's no such thing as Windows XP format or Windows Vista format. You should check however if your external hard drive uses an NTFS or FAT32 partition table. You can see that by right-clicking the drive (C, D, E, ..) and clicking properties. If it uses NTFS it will be no problem for Vista. If it's FAT32, then google for a way to convert it to NTFS. No big deal.
If you want everything works properly, then the answer is yes. Note: Yes
It depends on what kind of hardware you have. There is no direct upgrade from 98 to Vista. You'll have to basically format the drive and do a clean installation.
Yes, you can order the multiple set of Vista CD's from Microsoft after you purchased the Vista DVD, Windows vista comes on a single DVD and one recommended hardware requirements for Vista it's a DVD Drive.
You'd need to boot from some other drive and then use a utility to format the hard drive. One that you can get for free is called DBAN ("Darik's Boot and Nuke"), which will not only format the drive but completely erase everything that's on it. You'd then need to get some other installation media to install a new OS on the hard drive.