Windows Temp Folder

The .spl extension is for Flash programs and Print Spools. They're not essential to windows, but could still be needed.

If you want to get rid of temp files, I'd stick with running Disk Cleanup and also clearing them out of Internet Options.

you can delete everything in temp, this area is used for installing programs etc they are not needed and if they were they will automatically come back.

also do internet delete everything from time to time.