Windows crashing at specific times (Driver issue?).

Hello, I'm having problems with my windows for a few months now... the crashes started only while I was playing 3D games (I found that 2D games and flash games don't suffer from the problem)
when the crashes started it took some time to happen (around 1 hour) then the time until the crash happen become less and less as the days passed and now the crashes while I'm playing are happening
completly at random... and them the crashes started to happen also during the windows start up (not every day and most of the crashes happens before windows even start) and it also crashes around a certain hour

(around 19:15 and 20:30, but those crafts don't happen with often, note that I usually turn the PC On at 15:00)...

I already runned a complete scan with 2 anti-virus programs (Avast Free Edition and Malwarebytes Free Edition) and nothing whas found...
tried to stress the memory and nothing bad happened , tried using the windows driver verifier without success...
I've checked the event viewer but the last error in the lists are from months ago, and tried intalling "Who Crashed" with hope of find a fault driver but the program always point different drivers (when I'm trying to play it

says (usually) that a NVIDIA driver caused the crash or a third party driver that can't be identified caused it, or other drivers) the drivers most listed by the program are from AVAST!, COMODO Firewall, Windows

Drivers,or it says that a unknow driver made other driver (such the ones I mentioned) crash.

I forgot to mention that using any other kind of software (other than games) don't make the PC crash... it's almost certain that a crash will happen while I play 3D games...
also noticed noticed that my windows don't generates dump and mini dumps when it crashes.

Thanks fo the attention.
Sorry about my english.
what is the make, model and model number of pc??

It's possible that memory dumps are turned off...

1. Click Start, and then click Run.
2. Type sysdm.cpl, and then click OK.
3. On the Advanced tab, in the Startup and Recovery section, click Settings.
4. In the Write debugging information list, click Kernel memory dump, and then click OK.
5. Click OK to close the System Properties dialog box.
6. In the System Settings Change dialog box, click Yes if you want to restart your computer now. Click No if you want to restart your computer later.
ALSO, run checkdisk:
How to perform checkdisk

if you do not have Recovery Console installed, then run from the start menu

go to start, run, type in cmd

in cmd type in chkdsk c: /r and press enter

type Y for yes and press enter

type exit and press enter

restart pc, allow checkdisk to finish and run again

this is a lengthy process depending on the size of your harddrive, the percentages will fluctuate, this is normal, you can view report in the event viewer.
go to start>run> and type in eventvwr.msc, click on the applications directory, and in the right hand panel choose winlogon.

run as many times as it take until there are no errors found.

make sure to tell me of any errors received, exact wording no paraphrasing please
The PC is custom build:

FIC P4M800/P4M-800M/P4M-800T2

Intel Celerom D 315, 2266Mhz (17 x 133)

Transcend 1 GB RAM

Graphics Card:
NVidia GeForce 6200

performed checkdisk 5 times with no errors, but the scans took only a few seconds to complete.
checkdisk should take longer than a few seconds, did you use the /r parameter??

how large is the drive you are checking?

I have a 30gb drive, I use 10gb and my scan takes 20 to 25 minutes.
runned checkdisk again this time it took 35 minutes, but no error was found it just cleaned some non-used entries...

about the driver: I had that version installed but I uninstalled it and installed an older version because I thought that the crashes were that driver fault.
I thought my computer had a larger power supply than it actaully it has, it only has a 250W power supply but the Graphics card requires 300W.

above is from this thread, you might read it and see if one of the solutions work for you

the google search page here has several items for this issue, please read through several threads.

I am getting a little out of my depth here hardware wise, as my solution is to go with updated drivers, so if you do find a solution please post for any others who may have your issue, Thanks.