From memory, I don't think there is anything that allows control over it as it's such a basic application.
For what you are trying to do, I would look at writting a script that does the defrag (probably as a .vbs) and then use the CMD line version of the task scheduler - AT
Create put the .VBS in a file share, and create a .bat logon script with the AT command and relevant switches to run your .VBS.
I'll be honest, it's not very elegant, and needs a bit of thinking to prevent a new AT entry everytime a user logs on.
Windows XP doesn't have a CLI version of the defrag utility, however Windows Vista does. If it breaks EULA's or copyright or something then for the record I don't suggest it - however I would imagine that you could simply copy the defrag.exe file from Windows Vista, slap it on all of your workstation and then create a script against that.
Once defrag.exe is deployed to all your clients, you could then use the AT utility from your workstation to setup the job remotely on all of the clients. (If you have a lot then you can even script the creation of AT command locally and get it to do the work for you)
I haven't tried the defrag.exe utility outside of Vista, so I don't know if it works - however my personal preference would be to try that, as trying to get a GUI app that doesn't have a CLI to work using just a script can be a nightmare.
There's been a lot of talk about automatic defrag's on this forum and the WinXP one - maybe an idea to run some searches on there too.
Good Luck,
Steve.
"They have the internet on computers now!" - Homer Simpson