There is a builtin scanner that crawls/scans/searches the network and any shares or printers discovered are automatically added to My Network Places or Printers and Faxes.

The scanner/crawler is started in the following situations:

• At startup
• When opening My Network Places
• When refreshing My Network Places by pressing F5

The scanner is not started if one of the following conditions are true:

• If more than 10 computers are detected sharing folders (To be gentle on corporate networks)
• If using DUN or VPN connection
• If the scanner is disabled.

The scanner saves its findings here (If more than 7 days old then they are deleted)

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \Netcrawl \Shares]
[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \Netcrawl \Printers]

To disable the automatic scanning:

1. Open Control Panel
2. Double click Folder Options (Inside the grouping Appearance and Themes)
3. Select the View tab
4. In the Advanced Settings list uncheck Automatically search for network folders and printers

It is reflected with this DWORD value in the registry:

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \Advanced]
NoNetCrawling = 1

More Info MS KB276322 (WinMe)
More Info MS KB320138

Related Prevent share listing in My Network places