I personally posted an answer to this just a few minutes ago and I feel
my answer might help you our Sara so I am going to post it again on this
thread.
I'd personally recommend you launch up vista. Type in compmgmt.msc into
your start search. Select 'Disk Management' located under 'Storage'.
Find your main drive. Right click on it and choose the option Shrink' if
its available. Shrink it by 20-30gb. Now you'll have unallocated space.
Right click on the unallocated space and format it as ntfs. Grab a
winxp disc and install it onto the new blank partition.
If worse comes to worse and you can't get xp back on, at least you'll
still have Vista. Plus, you'll have the added advantage of dual-boot
when your all said and done. That way maybe 3-4 years down the line or 6
months (depending on your level of optimism), you'll have Vista already
installed when Vista's teething pains are over and until then have your
familiar xp readily available.
P.S. After you install Windows XP, you'll need to bootrec /nt60 /f all
to restore your Windows Vista bootloader from a vista setup > repair my
computer > command prompt. Once you done that, restart and you'll boot
into Vista. Once booted into windows determine which drive your win xp
shows as being installed on, edit your boot.ini on your win xp drive to
delete the part that says disk(1)partitoin(0) etc...and replace it with
X:\ where x is the letter of your xp install partition within Vista.
Open an administrative command prompt.
type in bcdedit /create {ntldr} /d "WindowsXPMyFavoriteBaby"
(Note in quotes you can enter any description you want to show on your
boot loader)
bcdedit /set device partition=X:\
Note x is the letter where your xp intall partition show as within
Vista.
bcdedit /set path \ntldr
Ta da! Your dual boot with Windows xp is setup. Nice, simple easy.
***Important note***
If compmgmt.msc doesn't let you shrink your partition, its cause
compmgmt.msc's partitioning tools in all honesty suck. In which case you
'get' by whatever means you choose 'Partition Magic' or a similar
program and shrink your partition that way.
Hope this helps you out
Wes.