First, Windows Vista does run slower than a comparable system running Windows XP in most cases. The main reason behind this is Windows Vista is a much larger operating system. It has more complex processes running and more of them to handle than the older Windows XP operating system. There are a lot of things that can cause a bottleneck in a system and it can be the small details such as a hard drive with a small amount of cache or a stick of RAM starting to go bad.
The second thing that can make accessing and loading files from your server slow is the actual network infrastructure. Now, I know that you said in general the whole system is just running slow, but more often than not a lot of the tasks that people talk with me about their system is running slowly is actually tasks where they are working on the network and that's what is causing things to run slowly. If you are noticing it run slowly trying to locate files on the network, copying files or saving files to the server, etc. then it might very likely be a networking infrastructure issue and not necessarily just an issue with the computer.
If you wish to run Windows XP on this kind of computer, you can without problem. The maximum amount of RAM that XP will recognize (unless you chose to install the specific 64-bit edition, which I would not recommend) is between 3 GB and 4 GB which is what you are currently running. However, Windows XP has reach the end of its life. Microsoft is no longer really supporting the OS as it's more than ten years old. Not only that but you are going to have a hard time finding the Windows XP for sale anymore and soon will be hard to find Windows XP compatible drivers. With the computer hardware you have, the system could support Windows 7. That is what I would recommend. It runs more efficiently than Windows Vista, is going to give you room to expand if needed in the future, and since it's a new OS it's no only more secure but also will offer higher compatibility into the future.
That is, if a completely new operating system is even necessary. Personally I don't really like the way that Windows Vista runs, but that might still be drastic to say you have to switch to a different operating system to fix the issue. There may be simple things instead that you can try without something so massive as switching the operating system. I'd suggest starting with a Disk Cleanup, as you'd be amazed how much stuff can be cleaned out doing this and it will make a noticeable difference in performance. If you've not done a disk cleanup before, it's quite easy and straight forward.
Open your start menu, and in the search bar type "Disk Cleanup". It should come up right away with the program option Disk Cleanup or Disk Cleanup Utility. Start the program, and select the drive you want to clean (most likely C
and let it calculate for a moment. It will then present a menu with lots of check boxes next to lists of items such as Temporary Internet Files and Recycle Bin. What I would recommend here is just to go down the list and make sure every option in that scroll list is checked. Then click OK at the bottom. It will go through, do it's thing, and once it's done the window will close automatically.