Dell Precision M90

I have the above laptop, now I don't know how old it could be, probably a few years, however I need to clean it up.

I need it to run one piece of software, and as quick as it can, the laptop will be used for nothing else. It is running on Windows XP, which the software is rated for, and through my research I have found that it has around 4GB of RAM, can I add more??? And will it have an obvious affect???

I have generally cleaned up the laptop, uninstalled everything, deleated all the documents, ran disk clean ups, defragmented the disks etc., other than that I don't know what else I can do.

Cheers,
Andy