Random black screen crashes

I am getting random system crashes. It does not happen on boot. Nothing specific triggers this, it might not happen for 5 hours or so...... but once it happened 3 times within 6 minutes, each time when clicking on a certain link. Mostly it happens when I click on a link but it might also happen when I do nothing.
When it does happen, the screen suddenly goes black, a brief message saying "No signal - going to sleep" will appear and then the system will boot itself up again automatically. After rebooting, a message will say "The system has recovered from a serious error - please report this to Microsoft ".
This problem started about 5 days ago, when I had to change over to a different ISP. I told the new ISP guy that I was getting frequent "Limited or no connectivity" balloons. He said this was due to a certain network security key, as my wireless network was being managed by the wireless USB adaptor software. So he changed my wireless network to be managed by Windows. The next day I had the first crash.
I have called him back but he says it is my machine that causes the black screen crashes, not the new cable modem or the wireless network management settings.
I run XP with SP3 and IE8. I have not installed any new programs.
What could be causing the black screen crashes?

 

Thank you, I have now added my system specifications.
I have also set up the computer so that it does not automatically restart on system failure.
The system failed again, and I got the Blue screen of death. The first line read as follows:
DRIVER_IRQL_NOT_LESS_OR_EQUAL (I assume this is the stop error code you are talking about)
No new hardware has been added except, like I said in my first post, they changed the cable modem one week ago, as per my Association, I had to change over to a different ISP.
Thank you - what is the next step I should take please?

 

I have just downloaded the latest driver for my graphics card. I had a driver that was dated 2003 - so I will see if the dreaded BSOD will come up again.
I will wait 2 or 3 days to make sure all is ok, then I will mark this thread as solved.
Thank you for your help, much appreciated.

 

The problem is back. DRIVER_IRQL_NOT_LESS_OR_EQUAL
The error stop messages are as follows:
OX000000D1 (OXFFBE8D86, OX00000002, OX00000000, OXF67F4810)
Help please!

 

Ummmhhh I cannot open the case and check the RAM physically. I am a female and not that far adanced, LOL. I did however replace the paging file and so far so good, no crash for 2 or 3 hours. Everything seems faster too, was a bit sluggish before. I also had a random frozen cursor problem from my wireless laser mouse, hopefully that'll be gone too. Will mark this thread as solved after 3 days if all is well.
Thanks again.

 

I have had nothing but problems, blue screens of death and frozen mouse cursors. Yesterday I could not get onto the youtube site, all other sites were no problem. I had to unplug the modem and plug it in again, then I could access youtube again. Very strange.
Today I realized that one of the blue screen error messages was related to my wireless network adapter. There was an ethernet cable connection between my new cable modem and the wireless router. I removed the ethernet cable and connected my desktop pc to the modem via a USB connection. All problems are gone, no more freezes, no more crashes.
Of course this is only a temp solution, as I cannot be online with my laptop now, eventually I will want the wireless network back.

 

That is the problem. I was told I do not have a built in network card, that's why I have a wireless network adapter, at least I think so. This network was installed by someone else who will help me get it right again.