System running win7 losing time on my Acer 8930G laptop

I was hoping that someone may be able to provide some advice re a small problem I have. Since my laptop died and I've rebuilt it. The time is not correct on it. When I power on the laptop the time is wrong and I then reset it to the correct time. If I leave it on for a few hours it seems to lose time if this makes sense. I believe the CMOS battery controls the time when the laptop is switched off and this could be the problem, but I thought that once the system was fired up the time was controlled by the operating system. Is this correct?

Also if it's the CMOS battery that needs replacing is there anyway of knowing this is the issue before dismantling. Problem is that the location of this battery requires full dismantling and reassembly. Typical manufactures they put the CMOS battery in the most inaccessible place on my laptop.

Would anything other than the CMOS battery be responsible for this isue, could it be a virus or anything along these lines..

Any advice appreciated...

Thanks Matt thats helped me understand the function of the CMOS battery..I'm afraid it's not a simple job but a complete disassembly which may mean it's cheaper to buy a new laptop. Acer have quoted circa £200 for this..so I'll see what options I have, I could try it myself as I'd have nothing to lose or see what an alternative 3rd party shop would quote to do the work.. thanks john..

So it appears that a gradual loss of time would be a first impression that the CMOS battery is failing? It's a pity there is no way to test the CMOS battery without having to remove it and meter it...