Major Problems with sp2 on e-machine pc.

installed sp2 with no problems. The install appeared to go ok, so when it finished, I turned off the computer.
The next time I turned on the computer, it did everything normally until it got to the desk top screen, before it finished loading, it rebooted 8 or 9 times, then went black and stopped responding. It would not turn off, or reboot, or anything.
After several months of trying everything that microsoft sugested,to no avail, I turned it on, it started,and left this message, System CMOS checksum bad – Default configuration used. Then it stopped responding again, rebooted itself and went black.
Turned it off, then on, it started, used f8, safemode, the screen blinked, and left this message, NTLDR is missing press Ctrl+Alt+Del to restart.
What do I do now, please, somebody help.