Just to try and stem the repetition of the "disable the QoS packet scheduled" tweak:
Microsoft says tweakers that are just guessing are wrong. Read the section "Clarification about QoS in End Computers That Are Running Windows XP".
The packet scheduler doesn't need to be disabled or set to 0 or anything. It only reserves bandwidth if an application with QoS functionality specifically requests that it reserve bandwidth, and it will then only reserve the amount the application actually uses.
I've never quite understood why everybody didn't just remove the packet scheduler from the networking setup during installation. I guess everyone just does the default install. Even if it's not doing anything, having it installed means just one more networking subsystem running.
I can tell you now that the R5300 does not have any bandwidth control features. (At the job I just left 2 weeks ago after nearly 3 years, I worked with this router regularly.) Everything connected to it is connected via an ordinary hub built into the router, not any sort of switched ports that can be individually controlled. The router also does not have any features which would allow you to control throughput based on IP address.
One option would be to use a filter set to restrict traffic on port 80 to the particular IP of this machine. Block all traffic on that port for that machine, while allowing it for all others. On one of the other machines, install a proxy server which uses another port, like 8080 (and if your network is using public IP space, only allows requests from the internal LAN) and has the capability of limiting throughput. Then set the machine you want to limit to use that server for all web traffic. (Any web applications which don't use the browser's default settings would also need to be manually configured to use a proxy.)
While this would mean that the "server" would have additional amounts of traffic going over its ethernet connection, given the actual amount of traffic available even at the max of a T1 compared to Ethernet, it should hardly affect performance for that machine even with a PC to PC file transfer, and it shouldn't cause too much of a performance hit on the machine. If you have even a slow machine spare, you could set that up without a monitor, keyboard or mouse, just sitting in a corner.
One other result of course is that all web traffic would appear to be sourced from the server machine's IP address, as far as remote sites are concerned.
The only alternatives would be getting a more full featured router, which would be quite expensive, or setting up a full-fledged firewall to control all traffic passing through your LAN.