I came across another forum that suggests that running an "HP Simple Save" backup drive is the problem. I run one of these, and it makes sense for me. Have adjusted my backup settings so that the external drive does not backup iTunes, I'm probably going to move my music backup to the cloud... crossing my fingers that this works.
http://h30434.www3.hp.com/t5/Other-HP-Consumer-Products-and/HP-SimpleSave-and-iTunes/td-p/542783
This question is a lot more complicated and detailed than the ones I see in your question and answers pieces. I have run this through every other source I can think of, so here goes:
I have a Dell desktop running Vista.
With stunning regularity,when I open iTunes, my library is empty. This sometimes occurs several times a week. Other times, my iTunes library will stay in place for two or three weeks, but rarely longer.
My library is still on my computer. I open my iTunes Media Library and pull everything over - music, movies, podcasts, apps, etc. - and the library is again populated. I have over 17,000 songs in the library, so this procedure takes at least half an hour.
Then, when I go to sync a device, I get a message that this device is synced to another library and all files will be deleted. (We have an iPad2, two iPhone 4s - opposed to 4S - and three iPods). The erase and sync procedure take at least another half hour. When I look at the list of devices associated with my account, it shows two iPods that were retired years ago, before I got the current computer.
I have been on the phone with both Dell and Apple several times. And I have spoken with people at my local Apple Store. I have deleted and re-loaded iTunes a few times. I have looked at numerous on-line forums. I find the occasion reference to this happening to people, but not with the regularity with which it happens to me.
No one has an answer. The best I got was an Apple Store employee who said the Windows version of iTunes wasn't very good.
This issue started when the computer was a little more than one year old. So it worked fine at one time.
Aside from the iTunes issue, my Dell works fine. I am not inclined to buy another computer. I just want to find a solution to this.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic