What my best solution?

We are a small property sales company that have recently implemented a LAN into our office for simple file and print services. Up to now we have been using MS works database for our customer/business details and this has worked very well. The file is stored on one of the PCs on the LAN but in order for another PC to have access the file must be closed. This system is nigh impossible to maintain as sometimes the office is very busy and one might forget to close the file. We are now at the stage where we want a database solution that can be open at all times and be 'live' updated. as we are P2P at the moment we don't really mind having to buy a file server but would like to save the money if possible. I have heard of MS Access and sql Server but I need more information, i.e.