I did rename that file (while it wasn't running) to something else, and proquota.exe reappeared on the next boot-up. I've seen Windows do that before with other stuff, so wasn't unduly surprised.
Not sure how you renamed the file but you may want to be aware of the following....
When renaming, moving etc files from explorer if it is a registered application explorer will handily (read as pain in the ass) update the registry to reflect the changes you have made.
The way to prevent this is to drop to a CMD prompt and do it manually at which point any links with the registry are no longer preserved.