I live in a two story very old house. The furnace is an oil burning and uses water to heat. This winter I can get the upstairs to stay near 70 but the downstairs barely gets above 50. I have been told there are two zones but that doesn't help me figure out why one zone heats and one doesn't. I can hear the water rushing through the upstairs pipes (a lot of noise) but only faintly (barely a tinkle) in the downstairs. I also checked the furnace tonight and the PSI is 5 and the temperature is 100 and none of the pipes are hot to the touch. Can I fix myself or should I get help?

Under the circumstances you posted you have more than one problem. The boiler is low on water, the temperature of the water is to low and you have a zone problem which will probably need a valve or motor operator replaced. It would probably be in your best interest to call for On Site service to get it all squared away.