Why would I want a 'business' broadband connection instead of a domestic one, if the former costs more and doesn't seem to give me much extra?
There are technical and contractural differences. Business connections can offer higher reliability, a better class of technical support, extra features such as fixed IP addresses and more guaranteed bandwidth. Domestic connections not only don't have these, but often explicitly prohibit hosting web or FTP servers, running certain services or using more than a certain number of PCs on any network connected to the service. Nonetheless, many smaller businesses successfully use domestic broadband.
Is there any difference between cable and ADSL broadband, and what about wireless or satellite
At the less expensive end, cable and ADSL broadband differ chiefly in their connection to your PC or LAN -- cable uses Ethernet, while ADSL is normally either USB or an internal PCI card. The very lowest cost option is wires-only ADSL, where you have to buy the modem and filters, the best value higher speed connection is the 1Mb service from UK cable companies. Wireless is the least common option, but works just as well; satellite broadband is more expensive and has much higher latencies than the others, and should only be considered as a last resort.
What backup should I consider for my broadband connection?
You can arrange for ISDN, dial-up or even another broadband system -- if you're lucky enough to live in an area served by more than one provider -- to take over your Internet connection if your main broadband connection fails. Under Windows 2000, NT or XP you can set up multiple gateways to the different services and allow the operating system's dead gateway detection to switch over on failure, or you can often configure your router to detect a dead connection and switch accordingly. Hot Standby Router Protocol, HSRP, as described in RFC2281, may be useful.
How do I share a broadband connection over the office LAN?
You can run Microsoft's Internet Connection Sharing (ICS) on the PC connected to the broadband line: that PC will then translate traffic to and from IP addresses of the other computers on the network. It won't do videoconferencing or intrusion logging, but is free -- and with Windows XP onwards, comes with a firewall. The downside is that this PC needs to be on permanently, and if it crashes the Internet goes away for everyone. The alternative is to use a router/hub that connects to the broadband connection and does address translation, firewalling and so on -- these are more reliable, much cheaper than and use far less electricity than a PC.
Can I run voice over IP over my broadband connection, or make telephone calls over the internet?
Yes, although the closer you want to make it integrate with the telephone system the more it costs. Numerous instant messenger clients now include voice, such as Yahoo! IM, Microsoft Messenger and ICQ, and may be suitable if you're managing voice communications between offices. Alternatively, you can make calls through your broadband connection to a third party, such as Callserve (www.callserve.com), which can be cheaper for international calls but won't accept incoming calls. Companies like net2phone (www.net2phone.com) make telephone handsets that plug into your router and use your broadband connection to link through to their telephony servers.
My firewall keeps reporting port scans through my broadband connection. Am I under attack?
Probably not. Every teenager in the world plays with port scanning software at some point, and you're just as likely to be checked as anyone else. If one address consistently shows in the logs, you can traceroute the address (run tracert from the Windows command line) and complain to their ISP. However, some ISPs are now port scanning their own customers to check for open mail relays and other network-threatening vulnerabilities: port scanning per se is a fact of life.
How easy is it to run a virtual private network over a broadband connection?
If you have two offices on broadband and want them to share the same LAN, it's very possible to do this with a VPN. Windows has the necessary software, but your router/gateway to the broadband connection will have to either support VPN passthrough -- where the router just lets VPN traffic through -- or explicit VPN service, where the router itself manages the connection. This may be more appropriate where all PCs on one network need to access the other network; VPN passthrough is fine if you've got just one PC that needs such a connection.