Do not back up programs. Programs can/will contain viruses and spyware and won't work right if you try to restore them later anyways, because the Registry entries will be missing. Back up only data. MP3s, movies, text files, documents, emails, video game savegame files, etc. Burn all those files onto CDs or otherwise back them up.

Get Windows patches. I don't know which ones you need. If you just download the Windows XP SP1 (Service Pack 1) you should be fairly OK. Burn it onto a CD. You can download it from the Windows Update site somewhere probably.

Physically remove any cables from your computer which involve the internet. This would be an ethernet cable or phone line. Then put in the Windows XP CD and boot your computer from the CD. If your computer is unable to boot from the CD, check your BIOS. If you can't/don't know how, then you can make a boot disk like RSL said.

If the setup program gives you an option to "repair" Windows, tell it no. If it says you can install it right onto your existing partitions, say no. If it offers to install itself alongside your existing Windows installations, tell it no. You need to delete all your hard drive partitions and create new ones (or just one new one that takes up the whole drive probably). Windows tries to make this easy, and I don't remember what it says exactly, so just play with it. You may get an option for NTFS or FAT32. You may get an option for "quick" format. It doesn't matter much what you pick, so long as you format, and Windows says something like "OH DEAR SWEET GOD ARE YOU SURE?!??!1?!/1?!1?!" and you say yes and you see a big bar fill up as your hard drive is slowly erased.

Then procede to install Windows. Once it's done installing, install the Service Pack, to somewhat protect you from instant automatic virus download. Then and only then, plug your computer into the internet. Then you will have to reinstall all your programs from scratch. You should use CDs for this. Or download installer executable files from the internet from scratch. You will have to install all your hardware. You will use CDS for this, or (recommended) download drivers from the internet.

Then you should be good for another 5 or 6 months.