iTunes propmts for apple ID and password every time is launches

No solution -- just something similar seen on the Mac side. I'll try on the Windows side (also 7 & 64-bit) later on today and loop back if I run across anything relevant.

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For whatever it may be worth -- my experience with that popup on the Mac side:

I updated to iTunes 10.5.1 via Software Update late in the day on 11/15. iTunes then loaded once the update was done. I could see in the background that iTunes seemed to already have my Apple ID noted in the upper right-hand corner where it normally is, which I thought meant that it 'remembered' me and had me still logged in. Nevertheless, in front of iTunes, was a popup window asking me to log in to enable automatic downloads. It looked legitimate and I did sign in, thinking that I'd use iTunes Match eventually and figuring that the popup was an opt-in bit necessary for either Match or the 10.5.1 update itself. Then, after signing in, it brought up the same popup a second time. Er... It didn't seem to have any different language than the first, nor did it have a warning saying I'd fat-fingered my login the first time or the like. So, gritting my teeth, I signed in a second time.

That's the last I've seen of that popup asking to enable automatic downloads or the like. I've tried signing out within iTunes to see if the same popup about enabling automatic downloads would come back when I tried to sign in again -- nope. I've also tried signing in with an intentionally incorrect password -- and at least the normal Apple ID signin popup that's part of iTunes comes back with red text indicating there was a problem. Furthermore, now I'm wondering what that enabling automatic downloads popup was about since iTunes Match is something you need to subsequently purchase for $25 a year ... and when I look within Preferences -> Store, the Automatic Downloads section shows all three boxes unchecked (Music, Apps, Books).

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Anyhow, I too am looking forward to hearing what others have experienced -- hopefully there's an answer to the loop you are experiencing that doesn't involve backing up and re-installing iTunes from scratch or something similarly drastic.

I reported a bug to apple so it should get fixed soon. Open iTunes...when it comes up push cancel twice. Sign out of store. Close itunes. Open itunes and sign in. Should be fixed.

Well, the answer is quite simple.

I just signed in one time and have not been prompted since.

Sorry for the delay in reporting back on my iTunes update on Windows experiment:

Finally got a chance to do the update last night. It never asked me to log in to enable automatic downloads. Um...

It's reaching a bit, but perhaps since I only have two computers authorized for my account, and when I did the iTunes upgrade on the Mac it asked me to log in twice to enable automatic downloads, what if one was for each 'authorized' system? Since I apparently got lucky and the setting stuck earlier this week, I didn't get trapped in the loop of having it ask you every time it starts -- and thus, when I later did the update on Windows, it never asked me at all. Pure guessing, all around.

avnyc11 -- I haven't used any of these settings yet to see how they impact sync, but I noted that in Preferences -> Store there are the checkboxes for Music, Apps and Books. Mine are all unchecked. I wonder if leaving them (or, well, the "Apps" one at least) unchecked will keep your system from automatically snagging your family's apps? That is, left unchecked perhaps you can still do wired or wireless syncing with your device and it'll stay isolated from any other apps on your family's other devices. If so, that'd be slick since once you do that sign-in thing to enable automatic downloads, I haven't seen a way to take the sign-in back, either in iTunes or when looking around at my Account info online.

Some of us have no reason or desire to sign in. There should be no reason why we should keep getting prompted for this, either.

I have the same problem!
Exactly as OP described. 😟

Very annoying.

I have the same problem. It is driving me insane. I have reported the problem to Apple as well. I have tried every trick in the book but can't fix it. It was only happening on one of three computers in my house. Other things (not worth mentioning) led me to believe my one computer was having serious issues. I reformtatted the hardrive and did a clean install of Windows 7. iTunes worked great for about three days. Now it is prompting me for the password two times every time I launch it. THIS ***** APPLE! Fix it please.

By the way, the reason it is not happening on the other two computers is that I haven't allowed them to update iTunes in awhile.

Same issue. Started on last version of iTunes, persistent through the most recent upgrade, today. Incredibly annoying. This is going to alter my decision on a new iPhone. Who needs this.