Why is my computer so slow?

i have a 5 year old Dell Dimension 5100 with a Pent 4 2.80ghz CPU w 504 MB of Ram. lately it has been running really slow and it makes a lot of grinding noises. my computer is linked to Verizon FIOs and i use the computer for the most part to surf the net and for online banking which is what i've ever only done. i was told that upgrading the ram would not help and it most likely has to do with my antiquated cpu which cannot be upgraded. i have tried using several registry cleaners and they do not seem to make any difference. is there a need for me to shell out several hundred dollars every 5 years just to surf the web.