I agree with sujithmd, how many users are accessing the systems. I am going to assume that there arent a tremendous amount of users since you have grouped everything into one system without replacement for 5 years.
In the server check out the task manager and see how much memory you are using vs how much you have. If your amount committed is under your 3GB then consider looking elsewhere for your performance issue.
Common file share issues:
Drive capacity/remaining:
What are you using for drives? RAID 5? How many drives and how big? How much free space do you have? The more people you have accessing the data at hte same time the more spindles you need. Also, do you have a defrag utility running like Diskkeeper Server Edition?
Network:
Have you upgraded any network hardware lately? If the server is 5 years old it probably has Gigabit, but if the NIC is just a 10/100 and you put a gigabit switch in then your server NIC may actually be a bottleneck to the rest of the network. ie: If you have a bunch of desktops with gigabit network cards tied to a gigabit switch but the older server only has a 100MB connection you may get some grief from that type of scenario.
As an example, one customer has a 5 year old IBM x-series with a 6 disk raid 5 array and 2gb memory servicing 180 users with file shares and they have great performance. The memory rarely breaks 800MB usage.
