Try Arena 3.0, I think it should let you do that. (But at times you may have to do some unconventional things to start a game, like flipping the board and telling the engine to "Move now".)
http://www.playwitharena.com/?Download:Arena_3.0
The only caveat is that Rybka is the only engine supplied with Arena that allows weakening. If you want to play against other weak engines, you'll have to hunt for them. There are a handful of them out there, but you have to know where to look (or who to ask). 
hello
I wonder whether it is possible to play against the computer after having set its elo, the game BEGINNING WITH A SPECIFIED POSITION.
I have chessbase and fritz, and I saw in fritz this is possible to set its elo at the beginning of a game, but not when I have a specified position.
I need id to train for some particular themes, but I don't want to be executed by a chess engine rating 2800 or more.
Do you know if it is possible in fritz, or in another program, for windows?
thanks,
olivier