Create a batch file,
schtasks /create /tn "sleep" /tr c:\scripts\sleep.bat /sc once /st %neartime% /ru System
the first line creates a scheduled task that runs sleep.bat as system user 1 minute from now. The second line logs you of.
In the sleep.bat file you need to add the following line which will send the PC to sleep.
In the first batch file you need to work out what time it will be 1 minute from now (%neartime%)
set curhh=%curhh: =%
set curmm=%curmm: =%
if "%curmm:~0,1%" == "0" set curmm=%curmm:~1,1%
set /a nearmm=%curmm% + 2
set /a nearhh=%curhh%
if %nearmm% gtr 59 (
set /a nearmm=nearmm% - 60
SET /A nearhh=%nearhh% + 1
if %nearmm% gtr 59 if %nearhh% gtr 23 set /a nearhh=%nearhh% - 24
if %nearmm% lss 10 set nearmm=0%nearmm%
This wont be instant but it will be automated.
Alternatively you could set you pc to lock when resuming from sleep, and then the sleep command on its own will lock the station (although this doesnt log you off, its close!)