The amount of data should have no bearing. Your imports have no bearing. It is connection of the databases with SQL and your clients.
Not having a domain and no WINS increases network traffic sometiems a lot. Probably not the issue.
If I were there. I would uninstall it after backing it and all the data up. I would uninstall SQL Express, then install SQL Server Workgroup Edition as suggested. Install it as the default instance and install AC to that same instance. With everything redone and having a much more robust SQL Server that can take advantage of the 3.2GBs of memory available, it may work much better. I am not sure how Jon's answer is viable (sounds weird, but we'll see), but if it does turn out to be a message issue which is taking up a lot of resources frequently, then you would have more resources.
