Exchange 5.5 high processor usage

Installed Exchange 5.5, NT SP4, Exchange SP3. PentiumIII 550, 256M RAM. I set the Internet Mail Service to manual startup. Everything works great until I start the Internet Mail Service: then processor usage goes to 100% and stays there, ultimately bogging down the server until I have to reboot it. Memory usage and hard drive usage are not affected. No single processor thread is the culprit, but (using Task Manager)the STORE.exe is at 8000K or more. HELP!