Just a quick question: are ISP's allowed to see one's OS? And if yes, how do they do it?

They could try to fingerprint it, using infos gathered from different ports, but if you have any kind of decent firewall (or router), then there's very little they can do.

A friend of mine is connecting through PPPOE via cable directly to the ISP's server, no routers, OS is Vista Business. He's a bit paranoid now...

Vista has a built-in firewall.

Not that I understand why he's worried. Even if his ISP knew he's using a particular version of Windows, then what?

It's not like they actually care. As long as they're making money (i.e. you pay them), they couldn't care less.

Exactly what I told him... He's fine now. Thanks for your input

They can still sniff your outbound UA string. And they can also get a good idea by what sites you visit or programs you run. Visit MSFN and Google for "Vista" a lot? Run certain applications or games that use known ports and are OS-unique?

Yeah. They can figure it out if they want to do so.

Also your web browser reports your OS as well, and they could easily keep that information and use it for metrics. If you've ever run any stat programs on a webserver, you can see it can see just about every OS.

i was just about to type this.
i can track the browsers that IP address are using.