click here for XP system requirements.
"64 MB minimum supported; may limit performance and some features"
This indicates that your system is at the extreme bottom edge of the specification envelope and as all your running software such as anti virus will also want its chunk of RAM, explains why your system is so slow.
More memory would help, but with your systems age unless you can pick some up off e-bay it's likely to be ridiculously expensive. You'd probably be better off removing XP and going back whatever you had before, be it W98 or something else less resource hungry.