Thanks Kemical.
I dont really want to wait more than an hour or two. (Time is money!)
Do you feel that I am just being paranoid about virus files getting through a re-installation of Windows 7 ?
Regarding partition size I am completely confused. I have no idea how many partitions I want or what the issues are - (although FWIW, I did read some theory that a small partition might force itself onto the outside of the hard disk and thus run faster!).
Worse, the dialogue called "Where do you want to install Windows?" is extremely clunky. And if you click on Advanced options, you get the chance to: delete, format, new, extend, load driver, refresh. So I have deleted all the partitions, but when I created a new partition, windows7 insisted on creating its own partition called "System Reserved" (100MB with 86MB free).
Do I need to create more partitions? What are the issues? (e.g. Can I readily change partition sizes later?)