Deja vu! This issue started with wm2003, seems to have been fixed under wm2003se, but came back in wm5. What is up with this? It's nonsense. Anyhow, it was a tough issue to solve in wm2003. The only thing that licked it for me was AlarmToday.
http://www.pocketmax.net/alarmToday.html . It's simply the best fix for this problem. You can also try the reg tweaks program tweaks2k2, which has a number of alarm fixes. All of these other fixes suggested didn't work for me including the all of the free little reg hacks out there. Try the AlarmToday trial and if it works, it's only a $10 program. Besides its fix, it's a very cool program. Actually, one of the most useful around.