Get more RAM. RAM is the easiest and by far the cheapest way to increase system performance. 512MB is a bare-minimum these days. RAM is cheap, easy to install (usually), and will definitely help. Especially on older systems, RAM modules will probably be very cheap, so get as much as your system will support.
With Windows, the system will just bog down after a while. It pollutes itself with temporary files which aren’t so temporary, registry entries, not to mention the likelihood of spyware, etc. RAM will definitely help, but cleaning up the system as others have suggested is also a great idea.
When I used to run Windows, I kept a disk with all the installers I needed, a text file with step-by-step instructions on what to install, and I’d just reinstall the OS every year or so. It took a couple hours, but it was worth it.
Also, try turning things off one by one. Turn off your virus thing, see if your internet performance improves. If not, turn it back on and turn off something else. Try booting into “Safe Mode” and see if there’s a difference. Create a new user and see if that user has the same issues. Uninstall anything in your system tray that you don’t use, or at least prevent them from running at startup. Trying all of these things will give you a clearer picture of what’s going on.