What up groovers ... my take on it (building up of a reasonably adware/virus secure system from a clean starting point)
Drop all your essential files (ie anything you want to keep) onto a CDR / DVDR. Gather your installer discs for EVERYTHING. Also download & burn various utilities - spybot, ad-aware, a good antivirus (not. bloody. norton. --- try AVG or Avast!, both very good, well regarded, and more to the point.. FREEEEE!!) and, if you don't have a hardware one (and if you find XP's to be naff, if you're running that), a firewall like Zonealarm... etc etc

Oh and partition magic perhaps.
And all your up to date drivers, of course.
Do the download/burn off someone elses clean machine if at all possible.
Also while there make a suitable command-line bootdisc that doesnt load anything from the HD. Floppy is ok, but bootable CD seems to be where it's at now (e.g. even my win98 install disc allows this..), though you might need to alter BIOS settings to make it work.
Basically the boot disc needs to allow you to 1. Wipe the hard disc completely without reading anything from it, at all, apart from the basic partition information, 2. Kickstart the operating system installation process.
I don't know what an XP equivalent might be, but FDISK on win98 is simple and reliable enough; needs updated for large discs though, and can't handle anything over 120Gb even then (or 250Gb, when split into two drives, neither larger than 125 gig). Alternatively, partition magic.
Being double sure its all backed up (double check there isn't a manufacturer-provided D: drive or whatever that has re-install and driver files on), wipe the lot in the shortest simplest possible manner. Reboot. Go into the utility again and create the partition table (the reboot isnt neccessary, but helps make sure nothing residual lurks), then format the drive or drives as appropriate.
If possible during the installation process, leave the computer disconnected from the net (is that possible when installing XP?). Load the operating system at the most basic possible level (with all your desired options turned on), then have a quite temp-file cleanup, defrag etc. Load all your drivers (and repeat clean/defrag). Then the firewall, antivirus, anti-spy/malware tools (another clean/defrag), office/etc, any other software (final cleanup etc). THEN plug your internet back in and install any stuff relevant to that, only once the machine is clean and protected.
Always use Spybot's Inoculation function, btw.