My computer is going crazy! PLEASE HELP!?

My computer was very much infected with viruses and trojans. So, since I haven't done it in a long time, I did a quick format and re-installed Windows XP. I must note that there were 2 partitions I saw before I formatted the HDD, one was 250GB, which I formatted and which had my operating system, and the other was about 8MB, which I didn't touch and left alone. After the reinstall of XP, I installed AntiVirus software, after which I saw errors, spelled in bad English! And my computer immediately rebooted. When it turned back on, the AntiVirus immediately started finding trojans/viruses infecting Windows system .exe files! Today, after 1 day, I turned my computer on, and the desktop is completely RED in color, there is a biohazardous sign that says warning. There seems to be some spywareMaster or errorRemover mysteriously installed on my desktop? When I click to change my desktop picture, it tells me desktop settings disabled by Administrator!