Each View in Track-It! will have different options. For an extreme example, if you were in Current View > All Printers, you wouldn't be able to activate an Operating System column. So, you may have to play with your views a bit, but I'll use Current View > All Computers.
In here, right click and select Customize, and turn on the OS Version column if it's not already active, then OK back to the grid. I prefer to turn off the Type column while I'm doing this to get a "flat" grid.
You should now see a column that shows the OS for your machines. In my case, I have some XP, XP Pro, and XP Pro Media Center versions listed.
If I only pick one of those, then I'll only see that one specific OS listed in the grid. So instead, I'll put a "custom" filter on that column:
Now, all I see in my grid are the various XP machines. I can mass select, and retire all of them at once.
When done, just click Abandon Changes to current view.