Have same problem with 96 Luguna 1.8. Dealer has given us code (for free though at first they wanted £15). This code has to be manually entered by pressing the immoblliser button next to handbrake. Follow sequence mentioned in other reply. Trouble is you have to do this each time and after 3 goes you have to wait 15 mins. Just to make it even more interesting we were give 2 codes by Renault and it's a case of trying one and then the other.
Also seen this post http://www.carsurvey.org/review_55849.html and another one which talked about water getting in the receiver unit via aerial. They fixed it by resealing around aerial and WD40 in receiver unit. Note this is the unit above the rear view mirror.
Dealers seem to want £300-£400 to fix which nearly value of the car. Hope this helps. Dumb design though to be fair have had the from new for 10 years and this is first problem.
Tony
January 2006