This is not a VIRUS.... nor related to any Network setup/settings, or Firewalls
Windows XP default uses "Simple File Sharing" in order to changed this go to.
1. double click My Computer or use Windows Explorer
2. Tools > Folder Options
3. click tab "VIEW"
4. Unchecked the "Use simple file sharing (Recommended)"
Just be sure you have the "File and Printer Sharing For Microsoft Networks" installed (services) found in your network informations.