I work for a small, rapidly-growing consultancy. We have a medium sized office network with around 15 Windows XP machines, plus a few printers, and a PC which we call the 'file server' with a large, mirrored hard disk in a RAID setup. The majority of our day-to-day work is done directly onto the file server, which is shared using Windows File Sharing so that all the PCs in the office can access it; this setup works almost perfectly. (Of course, we take regular backups of the file server which are stored off-site.)
The firm is growing; we have an increasing need to be able to access our internal network, but especially the file server machine, from remote locations. What's the best way to do this? We are located in the UK, and have a BT business broadband internet connection, which is pretty cheap. Is it likely that our contract with our ISP prohibits us from operating a server? If so, roughly how much would this cost us?
We also want it to be easy to use. We map the file server to the z: drive on our PCs in the office; it would be fantastic if we could continue to access the file server in the same transparent manner even when connecting from a remote location, that way all of our file paths (like 'z:\stuff\spreadsheet.xls') would still point to the right place.