I assume you've confirmed that the boxes are checked in Autocomplete Settings (sorry, but even the obvious needs to be verified - we want to cover all the bases). Can you tell from addresses if somehow all autocomplete entries got deleted (and those in addresses are only recent since that might have happened)? I also assume you know it's not working because it isn't completing forms where it should be at placed you've visited before - I would like to know if it is currently working on new entries. Can you find some site where you can visit and verify that after completing the form and exiting the site and then re-entering the site that the new forms are not being completed either? Is that all that isn't working in IE? History being saved? Cookies being saved and working? Able to access sites without problems and open links or embedded files without problems?
First, we're going to check out IE to see if we can find the trouble there.
Hopefully, one of these procedures will resolve your problems with IE. If not, then you may want to consider upgrading to IE8 (I'm not sure why you are still using IE7 - perhaps you have a good reason like some important site that IE8 won't let you access but IE7 works or maybe you're just used to IE7 or just haven't gotten around to it yet - but IE8 is a very good product with some nice enhancements over IE7 and is well-tested so there's no reason to be concerned about it being new (in fact, IE9 beta was just released and though I recommend against that at this point, I say that to emphasize that IE8 is completely developed and stable and can be trusted)). In a normal situation, I wouldn't so strongly suggest the upgrade, but given that you have Vista and can't uninstall IE7 without uninstalling Vista too (way too radical for the problem), this offers a way to get a clean new version of IE on your system and if the other options didn't resolve the problems, perhaps this one will (and I'd give it a reasonably good chance of working - it's certainly what I would do at this stage if I were in your position). If you decide to do so, get it from Microsoft and not some 3rd party vendor - get it at: Internet Explorer 8: Worldwide sites and be sure to choose the one appropriate for your operating system.
And again, don't be tempted by IE9 Beta - for one thing it has problems with Vista that trouble even some very experienced users and for another we don't need to complicate the situation so that we can't tell if a continued problem with Autocomplete is because there's another reason for the problem or if it is caused merely by the Beta not functioning properly because they haven't yet ironed out all the bugs. The objective isn't the upgrade, but fixing the problem you have.
Otherwise, download these programs, update them so they have current definitions, run FULL scans, choose to delete any malware detected (on those where that's an option - don't quarantine - delete), and locate the log of the results and copy it to the desktop so when you go to eventually attach them to another post they will be quick and easy to find.
1. See if you can download/run the MSRT manually: http://www.microsoft.com/security/malwareremove/default.mspx NB: Run the FULL scan, not the QUICK scan! You may need to download the MSRT on a non-infected machine, then transfer MRT.EXE to the infected machine and rename it to SCAN.EXE before running it.
4. Run a full scan from whatever AV program you are using.
3. Now post the logs (attach them) from these scans (each should create a log and you need only copy it and attach it to the post - to do so you need to Go Advanced on the reply) along with an explanation of the problem (including the fact that you have no direct evidence you are infected - unless you do other than this problem which doesn't seem to want to be fixed using conventional methods) in http://www.vistax64.com/system-security/ for assistance by an expert in such matters. We have some very good people and one in particular who is exceptionally well trained and a bonified expert in malware detection and removal. She or someone else will assist or guide you from there and then we can rule this out as the problem.
I hope one of the above procedures resolves the issue and that this malware approach isn't required, and that if it is the problem is found and removed and your Autocomplete feature begins to function again. If none of the above work and no malware is found (or what is found and removed doesn't resolve the problem), then please post back and we'll look into some other options to try to identify and resolve the cause.