1. Your ISP probably doesn't want you to use it because they want to make sure you have a proper connection without any possible interruptions. But your ISP is not
concerned about your computer's safety.
2. It is completely safe. The Windows firewall will protect you from much
more than if you didn't use it. Yes, there are some more firewalls out there with more options, but the chances that someone or some program will be able to penitrate through even the Windows firewall are very slim.
3. No. I would not recommend going without a firewall - even if it is Windows firewall. Like I said, the simplest firewalls still protect against a great deal.
If you're unsure about this firewall (which many people are), I'd suggest getting a free firewall. I use a free one called Comodo. It's really neat and comes with a great set of options and a nice layout.
You can get it at http://www.filehippo.com/
- just look under Firewalls and such and you should see Comodo.