A couple of things! How long since your last defrag and disc cleanup?
It probably wouldn't hurt to run Ccleaner... do the disc clean first and the run the registry cleaner... just remember to back it up when you are prompted to do so.
It would help to uninstall any unnecessary programs/apps first, though.... Ccleaner will clean up any remnants... then run Smart Defrag to compact the remaining data.
These steps will help speed up operation/performance. However, your specs are pretty low and a bit more RAM wouldn't hurt if you have it/can get some dirt cheap.
You make no mention of your graphics card, whether it is onboard or AGP/PCIe, whether or not it has shared memory. If it's the latter, your system will take a performance hit, particularly when using graphics intensive apps.
A cheap standalone card with its own memory would help, but it is an old machine in PC terms and probably not worth trying to upgrade greatly or spending a lot of money on.
Hope this helps and best of luck. 
