Hi to everyone!
I've just wanted to launch NI Update Service 2.3 because I noticed that for a while it didn't appeared so I launched it and a warning appeared telling me that NIPALU.dll wasn't found in the system.
I tried to repair it going to Programs and Characteristics/Software National Instruments -> and choosing the program you want (in this case the updater) but when a submenu from NI appeared displaying me three options, I chose the Repair tool but it didn't work. So I investigated...
I read in other forum (http://lavag.org/topic/17917-avast-deleted-nipaludll/) that a possible cause for this mistake may be the new version of Avast! Antivirus' trunk. And, for my case that was the problem: The new version of Avast had retained in, the file NIPALU.dll thinking it may be a virus.
Then, you just select the file and make it a "Global exception" for avast! to always ignore it and stop thinking it's a harmful file (before you perform all of this make sure you have NI Update Service closed). Then, you just open it again and you're done!