You can configure XP Professional Edition to allow network access with blank passwords by editing the local security password. Normally you will receive an error message unless you disable this option.

To allow blank passwords, open the Group Policy Editor (open the Start menu, click Run, type gpedit.msc in the Open field, and click OK.). Expand Computer Configuration, Windows Settings, Security Settings, Local Policies, and select Security Options. In the right pane, double-click Accounts: Limit Local Account Use Of Blank Passwords To Console Logon Only. Select the Disabled radio button under the Local Security Setting tab and click OK.