Computer can't find "iTunes.msi" file

Help! I've been trying to restore and sync my iTunes and 30GB 5th Gen. Ipod after my computer stopped letting me open the files from the shortcuts, to no avail. I have gone to the iTunes website and tried to update iTunes, but it only gets so far in the file transfer before it stops and sends a message that it can't find the "iTunes.msi" file to finish cleaning out the old files. I can now open iTunes if I put a CD in to import, but I get no recognition of the iPod when I plug it in, and I've used all 3 of my USBs. I get a screen asking what action I would like to take, but opening the files in iTunes is not an option. I just want to get everything working again so I can download some new music! How can I find or restore the "iTunes.msi" file the computer is looking for, and what do I do if it is completely gone on my computer????