I have heard that fragmentation programs for the older windows OSs have security issues so it's not a good idea to go online if you continue to use them. Anyone know exactly what that's about?
This is a new one on me also.... I doubt it very much (maybe the person who told you this was just trying to sound smart or be interesting?)
Truth is, you should defrag your disks if you do any amount of web surfing as, especially now-a-days, some sites will download temporary content to your drive in case you need it (so as to minimize access delays).
This content is usually deleted as soon as you leave the site, but such saving and deleting causes fragmentation, so it would be advisable to defrag regularly.
But like Eyesee, I don't see how a defrag program could cause security vulnerabilities (unless the defrag program being used was so badly designed it screwed up the firewall files laying your PC vulnerable?)
Here is a link to a Defragmentation Information Center that has a lot of information on fragmentation and how to deal with it via defrag
Look the info over and you'll see why I doubt a defrag program would cause the issue you mention.