I admit that I'm guessing at this answer but I think that I might have a solution. (I don't use XP and I'm the only person who uses this PC so I don't have to deal with 'administrator' problems but this problem still sounds familiar).
My guess is that your problem might not be directly connected with the recent changes to user privileges. I use Windows Me and, from time to time, it's managed to lose my log-in information for internet access. A friend of mine uses Windows 98 and has exactly the same problems. So, it's possible that your PC is submitting incorrect information when you attempt to connect to the internet. (I assume that your PC is set to 'connect automatically', i.e. you don't have to type your password every time you connect).
If my theory about your PC 'losing' your log-in information is correct (more accurately, it actually substitutes incorrect information rather than just losing the correct details), then you need to find the dial-up networking settings on your PC. In Windows ME, you go to Start > Settings > Dial Up Networking. XP has a nasty habit of making things far more difficult to find (which is one of the many reasons why I've refused a free, so-called upgrade to XP - which I absolutely detest). When you finally get there (use the Help files if you need to) you will see an icon for your internet connection. Right-click on the icon and select 'Properties'. Click on the 'Security' tab and check that your username is shown correctly. (Obviously, if it isn't, change it to the correct name). Your password will,of course, be shown as a row of asterisks. Delete this (because it's probably the cause of your problems) and re-enter your password. Click OK, then try to connect to the internet.
As I've said, there's a fair bit of guesswork in this answer, but this solution has worked for me and my friend on several occasions so I hope that it might also help you.
Chris