CMOS check sum error

Have you tried changing the battery? This is probably all I need to say at this point but I'll go further. Most motherboards tell you the voltage level of the battery in the bios but YMMV on that. It only costs a few bucks to change it out and you can buy a new battery in any store probably (usually CR2032 3v - in any battery section in a store - walmart, Fry's etc).
http://www.homedepot.com/p/Energizer-CR2032-Battery-4-Pack-2032BP-4/203979705
Nice big pic for you. Brand really isn't important.

1. Have you opened the box recently to install something?
2. When you shutdown your PC do you turn the PSU off in the back or something? Is there an on/off switch you're using ON the psu or are you shutting down the PC and then turning off a surge protector it's plugged into?

I just replaced my own battery after doing some HDD swaps and found I had my bios blown away when I turned the psu back on and booted the PC. I usually set their OEM full screen page OFF (like the big ASUS ad screen etc whatever brand) so I know when this happens and I like seeing the post info anyway. You can usually run with a dead battery, if you never turn the thing off via the switch (or surge) so it never resets on you. Without the power being totally shut off it won't reset (usually, without problems that is) since the battery isn't needed to be working in that case as you have continual power from the wall. I actually ran like this for a long time and just forgot when I did the HDD's to do the battery also...ROFL. I know how to set my bios up (by heart..LOL-I've built way too many pc's in my life) so it wasn't even a concern for me as I only really take it down to change parts.

3. How old is the board in this PC?

Assuming the battery advice didn't work:
4. Have you flashed the bios recently and are you on the latest version for your board? It could be corrupt, might try updating (flash bios), then after that disconnect power & clear cmos for 10secs or so with battery out (of course at this point you should have a new battery in hand to replace it since you're in there or already done as noted above), pop it back in then load setup defaults followed by saving and exit. Let it boot up once to get through a post, then reboot and go back into the bios, Change your settings accordingly and save and exit. See if they stick then. If there is no switch on the PSU, you can always shutdown and disconnect power to make sure the battery is doing it's job. This may be overkill for some (and I could add more steps and flash switches), but boards like Abit etc used to have a whole procedure to work right (with a list of about 6-7 switches during the flash). If you didn't load setup defaults and boot first then go back in it failed in some cases. I haven't had that kind of issue in a while but I'm not building boxes on new stuff right and left these days. It's usually the same model or two company wide these days rather than building/fixing home user pc's and seeing a ton of different brands/models yearly in Motherboards.

If you're switching the power off, just stop doing that until you get a battery and the settings should stick (and I don't know why anyone would do this anyway, windows shutdown is fine). So I mean just shutdown in windows (start shutdown etc, whatever) and don't switch off the PC in back or at the surge which would amount the the same thing as turning off the PSU (no juice flowing to the psu if you shut off surge, even without a switch). The only time my surge is turned off is if an electrical storm is on the way or something and I'm unplugging everything from the walls house-wide (well, stuff I care about...ROFL), or if I'm working on the PC etc.

I could go on, but we need more info on exactly what you're doing if the battery replacement fails. Logon isn't really enough. Also it's 6am so not the sharpest tool in the shed right now...LOL. My bet is battery, but you shouldn't see that unless you are killing power in some way when shutting down (either at surge or on PSU power off switch). Typical PC's still have power after a windows shutdown only. You won't be switching out the CMOS in any case today as it's most likely not removable and in that case you've got board issues if other suggestions here don't work. BEWARE of ESD before you go getting into your case if you're not a tech guy. Pull the power plug, and use an ESD wrist strap attached to metal on the case or something. I could write a bunch on ESD and workarounds (some sound hilarious but they work in a pinch & I really don't want to encourage that crap) but you should get the point. BEWARE of ESD!

Hope this helps