PC Crashes randomly with no error message!

Hi all!
I'm having this really strange problem on my laptop PC:
It's a Gericom Hummer P4 2,8Ghz that originally came with 256 Mb RAM and an extra empty RAM slot. These RAM modules were normal desktop computer DIMM so i got it an extra 512 Mb to tweak it (it really did work, it's a lot faster now). This enchancement came with a price tho... =-(
Every 1 out of 3 (on average) times i turn it on , ever since i added the new RAM it just crashes. The screen either turns full black, or filled with "jumbled" graphics and the computer freezes completely (Wich could be indicate a graphics adapter related problem, an On borad Intel Chipset wich uses shared RAM).
These creshes either ocurr 1-5 min after startup, or not at all. If i reboot the PC later it won't crash either!! I've used a RAM tester utilty for 24h and no errors of any kind were found. I've also switched the RAM modules on the slots, but i had no improvement at all.
I tought it could be a RAM cycle access problem, but my BIOS is very, very limited and i can't access those features. The PC has no floppy drive, so i can't even use TweakBIOS to get there...
Anyone ever experienced any similar problem?
Anyone got any toughts on it?
The solution might not even be what i have been thinking it is...
I really need help on this. I'm not an informatics expert, i just have some basic know how (fut i'm a fast learner tho).
BTW, i'm running Win XP Home with SP2.
Thank you all for any help you can provide me with.
Just let me know if there is anymore info that should prove usufull.

Stunts

 

Just to be clear you have removed the new 512 and the problem dissapears completely Also what happens when you only have the 512 installed,different brands of ram can be incompatible and theres just no way around it,have you tried increasing and decreasing the amount of memory the video can have access to

Welcome to the Forum stunts

 

When i remove the 512 the problem just goes away... But the slowness returns =-)
I'm going to try and keep only the 512 Mb module and see what happens...
How do i change the ammount of memmory that the video has access to?
That would be nice to try too!

Thanks!

Stunts

 

Should be an option in the bios

 

Hell... no way!
I can barely change anything on the BIOs... just little more then boot sequence... It's prolly the most uncomplete BIOS menu i've ever seen...
Oh well, i'll just try your sugesstion and see what happens...
Thanks again for your help.

Stunts

 

it's not a Gigabyte motherboard by any chance is it?

you can usually access its more advanced features by pressing ctrl+f1.

If this is the case let us know.

 

Unfortunateley my Mobo is not a gigabyte... It's an Intel Brookdale-G i845G.
It has hardly any BIOS options... anyone knows how to access any "hidden" features? Ctrl+f1 does nothing, i tried it just to make sure. =-)

Thanks!

Stunts