Sorry not to have gotten back quicker.
Here's what I would do as a "sanity check" on your system.
Restart your machine, but as Windows is starting up, press the F8 key - tap it repeatedly until you get a boot menu that gives you the option of starting Windows in "Safe Mode with Networking."
This puts Windows in a very minimalist configuration with only the barest set of drivers necessary to function. If you get in this configuration, and can then browse the Internet (safe sites only, such as google.com) then the most basic operation of the computer hardware is probably just fine.
I have found McAfee and Norton to be the biggest pains in the neck for an AV solution, such that I refuse to use them. I use Avira Free Antivirus, which is incredibly lightweight and simply stays out of the way until it detects a problem. I've used it for years, have it installed on every computer in the house and on computers for my mom and nearly everyone I can suggest. It's simple, its free, and it works.
Uninstall ALL other antivirus products before installing Avira. It's *never* a good idea to have multiple antivirus products running on the same machine.
If you think IE is glortched up (glortched is a highly technical word I made up about eight minutes ago), you can go to Tools-Internet Options and reset Internet Explorer to its default, as-installed settings, eliminating any possible add-ons, toolbars, or undesireable drive-by crapware add-ons you didn't want or know was there, like the Super Galactic Extra Super Search Toolbar With Ads Out The Wazoo!!!.
IPV6 is the later version of the networking communication protocol used by computers on the Internet. The current version that's been in use forever is IPV4. Newer Microsoft operating systems "conveniently" have IPV6 turned on (and sometimes preferred) by default, which means your computer might be trying to translate website names into addresses for the newer protocol which might not be supported by your Internet provider. That is, some ISP's are ready and can support IPV6, and aren't and can't.
I know of a large company that has struggled with this very problem, because they have not deployed all the IPV6 "guts" and some computers in their company just hang up and act funny until someone

discovered that turning off IP6 and turning on IP4 fixed the problem. May help you, may not - but since you're at least getting strange requests for usernames and such it would tend to make me think this isn't your problem.
I have grown so tired of Internet Explorer headaches I just can't begin to tell ya. I have become a big fan of Google Chrome (primarily) and FireFox (secondarily). I would download and install either or both of these and see if your browsing problems persist.
Lastly as an ABSOLUTE last resort, you can entirely reset your TCP/IP configuration on your computer with the following command from an administrative command prompt:
netsh int ip reset
Mind you, this gets your network configuration in about as close to an out-of-the-box configuration as you can get. BE VERY CAREFUL. All standard disclaimers apply, your mileage may vary, batteries not included, not responsible for loss of use, use at your own risk, may cause post-nasal drip.....and I say that just to offer the due diligence caution....esp. because problems like this are VERY difficult to diagnose in a hit-and-miss forum like this
Good luck.