Indeed, scheduling a boot-scan once a week (or something like that) is not possible (not even in the Pro version). However, if you're familiar with registry, you may be able to do it manually.
1. Schedule the boot-time scan, just as you want it.
2. Open regedit and export the value
BootExecute from the key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager(make sure it's really just the
BootExecute value, not the whole
Session Manager - you may have to edit the resulting .reg file). Save the file e.g. as
bootscan.reg.
3. Using Windows scheduler, schedule the command
regedit.exe /s (some path maybe)\bootscan.regThat could do the trick