I don't do MP, but I know network stuff. Your ISP is probably wrong, but I'll withhold until I know what you're using. Some questions:

1. What is the exact model, and full revision number (probably on the bottom of your router)?
2. Have you updated (flashed) the router BIOS, or is it stock?
3. Your OS, and service pack.
4. Have you manually adjusted any services (if you don't know what this means, you have not)?
5. Are you now, or have you ever been, running any Symantec products on that S?
6. What firewalls have you ever installed (even if deinstalled)?
7. Have you ever run anti-spyware on the OS?
8. Have you ever manually edited your HOSTS file (if you don't know what this means, you have not)?
9. Do you have more than one network card in yoru machine (even if it's wireless + wired, e.g.)?

I'm having the same problems when I join games with 4+ people, not when I host for some reason.

Here's my info-

1. Linksys Wireless G Broadband router (model WRT54G)
2. Stock
3. XP Home SP2
4. Not that I know of
5. Yes, Norton Internet Security 2004 (always turned off when I play online)
6. Norton Internet Security's firewall (always turned off when I play online)
7. Yes, Ad-Aware SE Personal Edition
8. Not that I know of
9. Nope, just one

Quote:

This is likely the problem. Microsoft recommends you use XP Pro for 5 machines.

All machines I install/reinstall for people must use Pro, as I personally won't even attempt to deal with the differences at the technical level, since the XP Pro OS itself has several THOUSAND (not a misprint) MS-confirmed bugs at the moment (most do not affect the home gamer, though). I have not tested XP Home in the network environment, so I must rely only on what others say, and I know of no one who is technically knowledgable and using Home themsleves. However, here is one link. You might google for more.

If you understand MS mis-speak, then here is their 'non-tech' entry portal warning people who network with more than just a simple internet connection not to use XP Home.

The simple answer: get XP Pro. Perhaps swap the license with another machine running XP Pro (many machines come pre-configured wiht XP Pro) which does not network more than a couple machines. You'll need your product keys (the 25-digit numbers) and need to reinstall XP on each machine. Do a fresh install, not upgrade. A great side benefit is that your games will probably run faster/better/more stable in the wake of a fresh install!

Well I can't really afford XP Pro right now, but I'll keep it in mind when I get my new computer. Thanks man.

Don't buy XP Pro retail. Buy a used PC with the XP Pro licence. For the price, you (in effect) get a free Junker PC. Then use your XP Home on the Junker that you buy, and use the XP Pro on your good game machine. Try e-bay, or local papers, etc.