Thank you for your great advices, Engram. I'll do exactly like you said.

I now what you mean with the mailbox thing. I usually don't download many houses but about week ago I downloaded one from Simantics -site. To my surprise, I ended having such an army of mailboxes (in all colours and flowery patterns) in my game that I nearly wanted to cry. While trying to get all of them out, I accidentally deleted the real one too. I was so angry that I just wanted to rant to the person who did the house and gave no warning... but I didn't.
I actually haven't put the downloaded mailbox in my game yet. It's just sitting in a folder I use to store files before putting them in. Saving the neighborhood and re-installing the game would bring the real mailbox back but it's such an big thing to do, that I'm not sure want to go through it again.