Do you have Spybot Search and Destroy? If not, download it and run it. Then, open its "Tools" submenu and look for "BHOs". This will report on all Browser Helper Objects in your system. Remove any that you don't recognize or use (You can click on each one in the list to get more info about it).
You may need to turn on "Advanced mode" to get to these tools.
Next, you might want to search for and download "LSPfix" and/or "Winsock LSP". These are utilities intended to repair your Windows Layered Service Providers -- a type of network drivers in simple terms. These can get corrupted when you install or install and remove firewalls, proxy software, adware, spyware, and various kinds of 'junkware' -- any software that uses or provides any 'service' to your internet connection, such as filtering, etc. You'll notice that Norton will have one or more LSPs in there; This is how it inserts itself into your internet connection so that it can block bad stuff.
Also, if you have, or can create a secondary Admin account on your PC, use it to find and delete the file
C:\Documents and Settings\<your usual username>\Local Settings\Temporary Internet Files\Content.IE5\index.dat
This file is the index for your Temporary Internet Files. If it becomes corrupted weird things can happen. If you delete it and then log back into your usual account, Windows will recreate a new fresh index.
And please upgrade to Windows XP Service Pack 2 -- You're risking a lot by sticking with Service Pack 1.
If your connection deteriorates further, you can downalod all of the above-mentioned programs and utilities on another machine, copy them to a CD, and load them on your affected PC from the CD.