No, there's no way to lock someone into only one ISP with Windows, and if there were, it certainly wouldn't be via the registry.
Are you sure you aren't just referring to using an AOL email address for logging into the Dell website? Which wouldn't really require doing anything with AOL, it's just what's used as a unique identifier by Dell.
Just going to need some more details, because what your describing just doesn't make any sense.
I have a dell dimension 2400 I purchased in 2004. No problem with the system. I registered with Dell at that time. At that time we used AOL exclusively via dsl. I now am using google chrome or firefox. From my computer I am only able to log onto the dell website if I sign on to AOL. If I try to log in from google or firefox I get an error message about website is unavailable. If I use my Acer laptop I can log into the website via google or firefox. The acer is on a home wireless network so there is no difference in the internet portal. I have been going round and round with dell and getting no where. My feeling is they do something in their registry locking into only one isp. Any suggestions would be appreciated.