Whenever I start up my pc nearly every time, it comes on, reads the drives on the black and white screen and just before it loads up windows it stops and come up with an error saying 'CMOS Checksum Error press F1 to continue or pres Del to enter setup and enter settings, rememberto save changes when exiting. Not sure word for word but something like that. Then if I go into the bios and re enter the time settings and date and then save settings, it restarts and loads into windows fine with the correct time. but then when I turn off my pc and then later load it up again the same error appears on start up, and if I press F1 and continue then the time is wrong in windows.
Two things it can be if it's a newish computer look for a jumper near the motherboard battery it will have three pins two covered by a jumper this may be on the wrong set of pins it may be set to disable bios so that it reverts to bios defaults all the time. If it is the correct jumper, Have a look at the motherboard Manual to make sure. If it's an old computer it will more than likely need a new Motherboard Battery
"and also a checksum is a way of identifing either a chip or pcb"
Er not quite. A checksum (quite well described by it's name) is a mathematical calculation on a set of data. When the data is read (eg contents of the CMOS chip), the checksum is recalculated, and compared with the original stored version. If there is a difference, the conclusion is that the data has become corrupted.
You're confusing terms here - TTL (Transistor-Transistor-Logic) is a type of logic design, CMOS (Complementary Metal Oxide Semicondutor) is a technology for making chips. There is nothing to stop a CMOS technology chip using TTL logic circuits.
How to shoot oneself in the foot in one easy lesson
The "Complementary" part of CMOS actually makes it part of the logic design - so that whilst both types use only transistors (as opposed to DTL logoc, which uses diodes) - the logic structure of CMOS is different to TTL.