Wiping HD tips

Okay I'm gonna wipe my HD this weekend and install a fresh copy of WinXp onto it. Now I need to ask a few questions.
1: what should I do beforehand considdering my HD is rather full and theres quite a bit of mal/spy/ad/etcware possibly a few trojans and the like.
2: how exactly do i go about wiping my HD? I googled it and was ran around alot i just need some straight forward answers :/.
3: To make my mind be at ease as long as I wipe the computer NONE of the files from the first format will be there right? no viruses etc etc.

i don't understand. have you your data backed up? do you have a partition? For your new OS I recommend a whatever-gig partition for your programs, games, and OS, and then the rest to your files. This will allow you to just copy the app data folder and reinstall whenever you like.

2 you wipe it by getting your install disk and booting from it.
3 only rootkits can remain after a format, but you dont have those.