Here's a quick-and-dirty batch file I made to add a reminder to the task scheduler. Copy it into Notepad and save it as something.bat , then make a link to it on your desktop or wherever.

@echo off
set /p MESSAGE=What do you want to be reminded of?^
^>
set /p ALERTTIME=When do you want to be reminded (hh:mm:ss)?^
^>
set TASKNAME=%DATE:/=_%_%TIME::=_%
set TASKNAME=%TASKNAME:.=_%
schtasks /create /sc once /tn %TASKNAME% /tr "msg * %MESSAGE%" /st %ALERTTIME%
pause

EDIT: I can't figure out how to make LessWrong put a blank line in a code block. There needs to be an extra blank line before each ^>

It prompts you for the text and the time to pop up the alert. It does have some limitations (you need to specify the exact time rather than e.g., "alert me in 30 minutes", and will only work for the same day), but if people think it's useful I can improve it.

It also needs you to enter your password to schedule the task. It's possible to avoid this by putting your username/password into the batch file, but that's obviously a security risk so I wouldn't recommend it. If you want to do so anyway you can modify the second-to-last line of the file to add the following text (replacing 'username' and 'password' with your actual username and password):

/ru username /rp password