Chaning processor

i have PIII system with slot 1 processor which 500mhz, i have 700mhz processor which i want to install in to 370socket, but when i tried to install 700mhz in to 370socket bois is recognizing but it is very slow and it is hanging, i thought it may be harddisk problem, but when to reinstall os with same configuration it is dead slow, while copying files it is hanging, so i removed 700mhz and again installed my previous 500mhz but i am facing the same problem, their are no options in the bios and on motheboard, pls help out