I had to reload my OS (10.5) on my computer. I didn't back up all of the files on my desktop.
They are now gone. Is there any way to retrieve them or am I just SOL?
It's not impossible to retrieve at least some of your data. But unless your doctoral dissertation which you're defending next week or something of similar value is among the data you lost, it's very likely going to be more expensive than it's worth to recover.
You could try this application or a similar one:
But your best chance of a nearly-complete recovery is to spend thousands of dollars for a professional data recovery laboratory to do the job for you:
If you want any chance of recovering the data that was on the drive before you erased it, you must stop using that drive immediately.