After you get the new comptuers and transfer you critical programs with data, because XP is no longer supported with security updates, I would look at installing a linux distro on them. Something like Ubuntu or Mint should run very well on these old computers.
Mint 17 Review
Ubuntu 14.04 review
Why would I do that? I doubt I'll ever be a Linux user. Everything I have is Windows based and has been for many years. The last thing I need right now is to learn a new OS.
I'll probably just keep the old machines around for a year or so in case something I need didn't make it over to the new machines, then recycle them.
I just ran defrag analyze. It said, as it usually does, that a defrag is not needed. I've attached the report. Fortunately, it was just under the very generous BP limit of 13KB.
This is usually what I get. However, I recently installed the Carbonite Mirror Image Backup feature. I see in the report that there is one 447 MB file that has 79,221 fragments. The mirror image backups take 8-10 hours to complete. I doubt that this one file could be causing general performance problems, but it might have something to do with why the backups take so long. Although, they were pretty slow right from the start.