Windows XP doesn't defragment a disk automatically. You have to do it manually or set up a task for that. See:
- How to Automate Disk Defragmenter in Windows XP
- How do I turn off fragmentation in Windows XP? (sic)
Yes, a user can still run the Disk Defragmenter tool on Windows XP. If it doesn't happen to often, it doesn't do any damage. But it should not be set up to run automatically.
When you replace the disks with SSD ones, you should tell your customers that they should not use the Disk Defragmenter tool any more.
If you are really paranoid, you could delete the Disk Defragmenter tool from the Windows XP machines.
Windows 7 and later detect an SSD and refrain from defragmenting when not necessary. See this article. Conclusion:
No, Windows is not foolishly or blindly running a defrag on your SSD every night, and no, Windows defrag isn't shortening the life of your SSD unnecessarily. Modern SSDs don't work the same way that we are used to with traditional hard drives.
Yes, your SSD's file system sometimes needs a kind of defragmentation and that's handled by Windows, monthly by default, when appropriate. The intent is to maximize performance and a long life. If you disable defragmentation completely, you are taking a risk that your filesystem metadata could reach maximum fragmentation and get you potentially in trouble.