The Asus n81vg notebook came with vista preinstalled. I want to remove Vista and install a new version of windows xp home edition. This is a new product full version purchased for this purpose. Thank you

I have a laptop I want to sell, and I want to do a clean install of Windows XP and reinstall Office 2003.

1) How do I do a clean install? I’ve never done one before.

2) Would the product key for Office still work? I’m under the impression that if you use the key once it’s no longer any good. Am I incorrect?

3) I have the CDs for XP and Office, but they came from a different computer (this laptop had everything preinstalled and did not come with CDs). Does it matter if I’m using discs from another comp?

PS: All my software is legitimate.
If the XP disc is OEM, how would I go about the clean install of the OS?

Not only that, I need Virus Protection, anti spy, and firewall. I've been using the suite that is included with ATT Yahoo DSL but I need a different one because I purchased a new computer with Vista and they dont support Vista (and won't until at least August). Preinstalled on my computer I have a free trial of Norton but I was not happy with them on my last computer because I felt they take up too much memory. Give me some suggestions and why. I need the works but they don't have to be from the same provider. I was leaning toward computer associates but tell me what you think.