Insert Win XP disk, boot with it, format and install XP.
Remember that formatting erases all data in the HD you're formatting.

The Win XP install CD is pretty easy and self explanatory to use.

@BigPeBe
Thanks for the info. That's quite scary tho, every necessary software installed yet some malicious code can still get thru all of them. I myself have Nod32, Kerio personal firewall and Spybot S&D installed and luckily no infection so far

Now I have only Antivir and XP's own firewall. I used to have Antivir + Zonealarm + Spybot + Ad-Aware, but they used too much memory for my taste and especially Zonealarm made my Windows boot too slowly for my taste... So I compromised, because at least for now Antivir + XP's own firewall have seemed to be enough, managed to keep my computer quite clean with these. Sometimes I run some scans with few other programs tho.

From a personal perspective, I use Avast, my router firewall (and it IS a firewall, not just a NAT.. I have to actively block sites from EmmyLou so that she gets on with work instead of surfing the interwebs!! ) and Windoze Defender...all free, all work, and so far, virii and trojans and malware are things that OTHER people get...not me