If you want to schedule a Spybot scan under Windows XP without setting up a login password, the procedure is as follows:

For Pro: Start/Control Panel/Administrative Tools/Local Security
Policy/Local Policies/Security options. In the right hand pane (third down)
Accounts: Limit local account use of blank passwords to console logon only.
This is enabled by default, disable it.

For Home: Go to Start/Run/Regedit
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
Value name: limitblankpassworduse
Type: REG_DWORD
Data: 0 (disabled) 1 (enabled)
It is enabled by default. Disable it and scheduled tasks should run.