If a virus is replicant (coming and coming again), you should:
- Disable System Restore on Windows XP: http://support.microsoft.com/default.aspx?scid=kb;[LN];310405
- Clean your temporary files.
- Schedule a boot time scanning with avast. Start avast! > Right click the skin > Schedule a boot-time scanning. Select for scanning archives. Boot.
- Besides ewido, try a-squared or Spyware Terminator (trojan removers).
Other option is scanning in SafeMode (repeatedly press F8 while booting): http://support.microsoft.com/default.aspx?scid=kb;en-us;315222