It would be easier to install Windows on a separate drive. Then, after you install your AV and other protection, connect your original drive as a slave, boot up, and copy your files to the new drive.
You could use almost any size drive larger than around 60 GB, as long as there's enough room for the files you want to transfer after installing Windows. Once you have that together, if your original drive is bigger, and you need the space, you can use Acronis True Image or an old version of Norton Ghost or to clone the new drive back to the original one.
Before you transfer your files, you may want to boot to the new drive in Safe Mode and scan your old drive to make sure there's no lingering infections that could damage your new installation.
Western Digital and
Seagate/Maxtor both give away a free version of True Image 11. The only requirement for using one of these is that at least one of the drives (source or target) must be manufactured by company who sourced the version you're using.
BTW -- This version of True Image works to clone Win 7 drives, as well. So far, I've only done that between two matched drives. I've never had any problems cloning between mismatched drives on XP.
Hope that helps.
🙂