I'd say: some Windows Server (that's an OS) machine with the possibility to remotely login in a VPN. Don't do this yourself, but find a company that does this for you.
Discuss with them if it's better to have the server in your office or in some data center in the cloud. if the cloud, let them take care of setting up that also.

A very good alternative (I'd even say: a very much preferred solution) is to use some an accounting package that is offered as a cloud service by some company, in stead of installing your own accounting software on your own (physical or virtual server).
An example I happen to know is http://www.exactonline.com/, but I'm sure there are many. Maybe even the company that wrote the program you use at the moment offers such.