Deleting old dll-files?

When is it safe to delete old dll-files? With Duplicate File Detective, I've discovered that I've got som 820 MB worth of dll.

Are they not supposed to be touched under any circumstances, or can I start with removing those who at least three years has passed since the last access?

>>1

If in doubt, move them to a temporary folder one by one, restarting each time.  That way, if it crashes, you can use a recovery disc to put them back.

Also be sure to remove any reference to them in the registry so Windows does not think they are still there.