Sorry MB, I still havent had time to write the dual booting procedure down for you
First of all he needs to partition his new drive so that he has room to transfer whatever he wants from his current drive to the new one.
No point transferring the windows folder and contents as that wont work although he may want to save his emails.
No point copying installed programs either as they wil lneed reinstalling.
So he wants his documents, pictures, videos and music etc.
I assume that he has it showing in Disk Management now as an uninitiated drive ?
If so he needs to create and format a partion for his OS, I would suggest 40 gig for XP or 60 gig for Vista.
Then he needs to create and format another partition in the remaining unallocated space on that drive.
This is the partition that he should save his stuff from his original drive to, I usually make a folder on it called Old PC or similar as invariably there is stuff that he will want to delete later as we all save far too much.
He now has to decide whether he is using his 250 gig or the 500 gig as the XP drive.
He needs to install XP on one of the drives as the first OS, then Vista on the second one.
Assuming he is using the 250 gig drive for XP then its safer to disconnect the data cable from the 500 gig drive so he doesnt accidentally format it and lose the saved data.
Ideally he should be using a basic PS2 keyboard so that all the keys work without the OS being installed, USB ones can sometimes not allow the keys to work when installing.
Set the bios to boot from the CD/DVD as first device.
Restart with the XP CD in a drive and when it asks you to press any key to boot from the CD press any key
Follow the on screen instructions etc and when it comes to the part where it asks where you want to install to there is an option for deleting a partition.
Select this option "D" and press enter then to confirm I think it asks you to press "L" and then press enter.
I assume that you will have 2 partitions on that drive of around 120 gig each showing as 120000mb plus another one showing as 8mb.
Delete all the partition except the 8mb one by repeating the same procedure.
Tnen it should show as the full size of the drive but with no drive letter.
Then press "C" to create a partition.
In the next window it will show a box with the maximum size of partition that you can have, clear the numbers and for XP enter 40000mb that will give you just under 40 gig.
Select the format NTFS Quick option and when its done select C as the partition to install windows on.
Ignore the other partition for now and follow on installing XP by entering the information when asked.
It will reboot a couple of times as its installing but
do not press a key when you see the Press any key to boot from CD message as it reboots or you will go round in a loop.
Once windows has installed properly then go into Admin Tools/Computer Management/Disk Management and you can click on the unallocated partition and create a new primary partition of the maximum size it allows, ie dont need to change the numbers in the box and again select the Format NTFS Quick option.
Shut the pc down and connect the second drive data cable again.
Have the Vista DVD in the DVD drive and follow the instructions I sent you before and at the part where it asks you where to install it make sure that you highlight the 60 gig partition of the 500 gig drive.
It will want to format it again just select the NTFS Quick option.
The rest of the installation is as I explained to you before.
Once its finished installing remove the Vista DVD and restart the pc and go into the bios and select the XP hard drive as the first bootable device.
When you restart it should ask you which OS you want to boot into so just select which one and press enter.
Hopefully thats all there is to it
Thats the traditional way to dual boot by having the earlier OS installed first then installing the later OS afterwards as the later one understands the older one whereas the older one may not understand the newer one.
I still think that the best way to multi boot a pc with 2 or more SATA drives is to just thave a single drive connected and install one OS to that.
Then disconnect that drive and connect the second one and install the other OS to that one.
Then connect both drives and its done.
It doesnt matter which way round you do them and you have no risk of installing to the wrong drive or partition and thereby losing saved data.
There is no problem removing either drive at a later date as each OS is totally independant of the other drive or OS.
You just use F8 when starting the PC to tell it which drive to boot from.
This wasnt as easy to do with IDE drives as you had to enter the bios each time but with SATA drives F8 gives you a simple option of boot devices.