to put the pc onto standby does not require a button that you need to physically push with your finger. The way I put pc onto standby is click on 'Start' then 'Shut down', that should bring up a box where you can select what you want to do - shut down the pc, restart the pc, or put on standby. Note that i use windows 'classic' mode, which may be slightly different to you.
Another way is to press the 3 keys simultaneously ctrl + alt + delete. That should bring up a menu that includes an option 'Shut Down...'. click on that and it should present you with the options to either shut down the pc, restart the pc or put the pc onto standby.
Again because I am using windows classic mode, you may not get the same options. What you can try is change your windows xp interface to 'classic' mode and see if you can put the pc onto standby in classic mode.
Also, in the control panel, look for something that says 'power options'. open up that and click on the 'Advanced' tab. that should give you options for different ways to put the pc onto standby mode e.g. an option to press the power-on button and put the pc onto standby by doing that.