How to limit bandwidth usage by each PC on the network ?

I think a better solution would be configuring Quality of Service (QoS) on your router... rather than installing an app on each computer. What kind of router/switch are you using? They generally have an easy-to-configure quality of service option where you can limit the bandwidth of each port.

Before I make my suggestion for hardware, can you please tell us how man PCs you intend to have connected?

BTW, that add-in is indeed not compatible with the latest versions on Firefox; however, you can download pretty much all of the older versions here:

http://www.oldapps.com/firefox.php

That is, if you're willing to downgrade. Doesn't make too huge a difference in my opinion.

To block youtube, you can try editing the windows hosts file (but I'm not sure if that works with firefox, I am sure it works with IE) or configuring adblock, a firefox add-on

https://addons.mozilla.org/en-US/firefox/addon/1865

But seriously, I would not block such a popular site as youtube if I were running an internet cafe. =/

I would look into perhaps getting a Linksys SRW2016. It looks like it has what you need and is a 16-port switch to allow some expandability. Its about $200. Does that fit your budget? If not I'll keep looking, and I also need to look into it a bit more to make sure the features would work well for you.

This plugin should work with all versions of firefox from 1.5 to 3.6a1pre. If you have a older version you could edit the firefox throttle add on and see if it will work with your version of firefox before you look at a hardware alternative.

Uninstall the add on. Go to the link http://www.uselessapplications.com/en/Application/FirefoxThrottle.aspx and right click on the download and click Save link as.

Open the download with winrar and extract the entire file but leave winrar open. Go to the directory that you extracted the file to  and open install.rdf with notepad (right click the file and click open with).

In the file you will see the following entry:

 em:minVersion="1.5"   em:maxVersion="3.6a1pre"

edit either the 1.5 or 3.6a1pre so that it covers you version of firefox.  Eg. if you have version1 then change it to

em:minVersion="1.0"  em:maxVersion="3.6a1pre"

then save the file. Drag this file from the extracted directory back in to winrarand overwrite the existing install.rdf

Then close winrar, double click the xpi file and it should install. It may not work with your version or be able to limit the bandwidth enough for you, but at least you can be sure before you spend money!

@semtex: I realize the firefox page says versions 1.5 - 3.6a1pre, but I checked for the author and tried it myself. I am running Firefox 3.5.2, and the add-in installs, but after Firefox restarts, it will not start the add-in as it says incompatible with rv. 1.9.1.2

Also, if you look in the comments, many people have stated that this app needs to be updated to work with current versions.

You could try NetLimiter 2 Pro, it is $30, but there is a 28 day trial.

http://www.netlimiter.com/

Apologies, I think netlimiter may be a single host app only.

http://bandwidthcontroller.com/index.html this is a windows app which should provide what you are looking for.
Personally I would be agree with MaestroDT (sorry missed the comment earlier), and use a router with QOS. The router will also give you the ability to block ports (stop p2p programs) and a hardware firewall.

In addition, having a hardware-based solution will really increase performance. I mean, how do you have everything hooked up right now? To a hub or switch?

Allow me to summarize the benefits of a router or switch for this purpose:
1) Administer QoS/bandwidth limitation from ONE central location rather than each individual PC
2) Port filtering capabilites
3) Hardware firewall
4) Increased performance overall - routers/switches are made to handle lots of traffic. ICS is a cool idea for maybe 1 additional PC, but not beyond that.
5) Some routers even come with "parental controls" that will allow you to block whatever websites you want very easily.
6) Enhanced control over your network - you could easily keep the cafe PCs on a different subnet than your business PCs.

I'm sure there's more reasons, but that's a good start. That easily makes it worth even a $300 or $400 cost.

Be aware that a lot of the SOHO (Small Office Home Office) routers may claim that they can handle large numbers of connections although they are not really designed to do so. 25 users is quite a lot on a small router and this could fill up the internal routing tables and have a negative effect.

One option is to look at the possibility of a second broadband account and another router when you expand, although with a good router this will probably not be necessary. As MaestroDT says the functionality of a good router can easily make it worth $300 - $400  

I would be tempted to contact cisco sales and describe what you want to achieve and your budget and look at what they suggest. You dont have to buy it from them you may even find a second hand one on ebay, but from experience cisco make some very high quality and reliable routers and they live up to their specifications.

Yes, if you can stay within Cisco, please do. Linksys is fine, since Cisco is their parent company, and might be a little bit cheaper -- Cisco devices are MUCH more configurable, but that also sometimes means they require more time to setup, whereas most linksys devices take about 5 minutes.

I agree with calling up a sales rep and describing what you want. The main thing you need to think of is the number of ports. For QoS to work properly, each computer has to be connected to its own port on the router/switch. This means you can't buy a 4-port router with QoS, connect it to a 24 port switch, and have the computers hooked to the switch -- because then several PCs will be sharing the same QoS port and that just will mess things up for what you're doing.

Shop around a little bit. We've given you the guidelines for what you should look for. Also, there is absolutely no reason you can't get a product answer from Cisco Support and then buy the item on eBay for much cheaper -- in fact, I recommend you do!

Good luck!

-DT