I suggest:

  1. Disable System Restore on Windows XP: http://support.microsoft.com/default.aspx?scid=kb;[LN];310405
  2. Clean your temporary files.
  3. Schedule a boot time scanning with avast. Start avast! > Right click the skin > Schedule a boot-time scanning. Select for scanning archives. Boot.
  4. It will be good if you download, install, update and run other trojan remover tools (like the ones suggested by Frank).