Since you have vista on your system i would bet that there is another partition that is hidden from your windows observations. This would be the Vista restore partition.
Installing XP would be the best way to accomplish what you are trying to do.
But before you do anything: download this tool and use it to identify all the hardware on your sytem and then print the results or write down the following info from the program:
motherboard model #
Graphics card model
sound card model
chipset manufacturer and model
networking model #
and any other Hardware that is built into or added to the computer. Especially any SATA / RAID controller drivers that are listed. You will need these DURING the installation proccess so read all instructions from the manufacturer's site concerning the proper installation media (floppy/CD) you will need for your computer.
The tool will also allow you to find any software licens numbers for easy replacement of installed software.
The stand alone version is all that you need: SIW | Download
Don't fret as most of the drivers that you will need to reinstall will be readily found at your computer manufacturers website under support > downloads. It might be wise to download the drivers and burn them to CD before you start. That way they are available as soon as the installation is complete...and ready to be installed.
Make sure the Boot Order is set to CD as first boot device. This can be done in the BIOS or with some computers by hitting the appropriate key as shown during the boot sequence.
Insert the XP installation and restart the computer. At the prompt "Boot to CD...press any key" hit enter immediately. If you miss this you have to restart your computer (once you have booted to the CD you do not need to do this again no matter how many times the sytem reboots itself)
Set-up will begin and then you will be prompted to install any third party drivers by pressing F6 (only press this if you have sata harddrives or are planning on using a RAID configuration). If you choose this option you will be prompted to install the drivers at this time. The next item is to accept the End user license agreement by pressing F8. Set-up will then ask to repair or set up XP...choose to setup XP now.
Set-up will now find all harddrives (partitions) on the computer including the hidden one for vista. Select each partion by hi-lighting it and choose to remove the existing partition. Follow the prompts for removing each partitionuntill you have only a single large partition.
You need to make a decision as to what you need to do...I reccomend that you make a 40 gig partiton as your main partition format to NTSF and install the XP operating system to that. Afterwards you can format the remaining "unallocated space for storage/backups whatever. If you wish to have the whole drive as one partition then just choose to set on it. I reccommend that you use the full formating program instead of the fast format.( better at cleaning the trash out)
The installation will procced and just follow the prompts as indicated.