Parity Error

Im having an Dell Optiplex XM590 Pentium system with 90 mhz with 16mb ram ,later on i upgraded the processor to 100 Mhz and Ram to 64 Mb 16 x 4 Edo’s now i installed win98se os in my system.. sometimes im getting an error message “Parity Error” System Halted, i suspect it may be problem with RAM and when i created a swap file its i mean viritual memory its fine..what is the prob and why im getting the Parity error as and when i get the error theres no way i need to restart the system how can istop it.