Freezing!!!!!!

OK, i kown it's been posted hundreds of times, but i took the time (quite a lot) of reading every post regarding this issue and it hasn't been fruitfull... so I decided to post.
My problem is this:
Every time i turn on my computer it loads perfectly. Then it reaches XP and it lets me work for a while... But at some point (no more than 5 minutes) the screen freeze and i'am not able to reboot using ctrl+alt+del.
After this, i reboot the sistem "fisically" an Xp loads again. The same thing Happens.
So far, same issue everyone has asked about, but here comes the STRANGE part:
usually the third time i load XP the pc works just fine. I can use it for hours without any problem. I can even reboot again and it will work just fine...
This logically leaves off the COLLING PROBLEMS. Plus I already run it with the case open, and i have "felt" the heat inside is normal.
I thought then it could be a VIDEO CARD issue, however I've downloaded the last drivers from ATI.com and the problem continued.
I also thought it could be RAM issue (i only had 256mb) , so i bought 512mb more. Nothing.
Finally i decided to backup and reinstall de pc. After i did this I thougth it had worked. But it didn't...
I don't think it's a virus issue because i'm using avg 24/7 and i've already ran more than one system scan.
I've also run the event viewer, and although there are a couple of error they don't seem to have anything to do with this.
I thought about updating the bios, or switching to the on-board card but i don't know... maybe it's something easier that i'm not seeing that someone sees.
these are my system specs:
Asrock K7S41
AMD Athlon Xp 2400+
768 MB RAM DDR333
ATI RADEON 9550 256mb
Western Digital 30gb ATA
SONY CD-RW
WIndows XP SP2 + all crytical updates

HELP!!!!!!!!!!!!!!!!!!!!! thank you

Is this a new problem? Were things working OK previously?

A freeze without error is generally a hardware issue.

If it is not heat then the graphics card could be suspect even with good drivers.

Also the PSU is a prime suspect. How old is the computer? What wattage is the PSU?

Did you Clean Install XP? This would be the only way to totally eliminate an XP issue.

You might try running chkdsk

start > run > cmd
in the new window type chkdsk /f
when prompted to run on boot choose Y & reboot
this will check for & fix any disk errors.

As to hardware I would use your new 512 RAM by itself in the first slot.

Remove everything else but the CPU; RAM; Graphics Card; Drives.

Another test would be to run in Safe Mode and see if you have any problems:

Try booting into Safe Mode by repeatedly tapping the F8 key after your BIOS POST but before Windows starts.

Also view your Error Log via:

[1] Start Menu
[2] Help and Support
[3] Pick a Task
[4] Tools
[5] Advanced System Information
[6] View the Error Log

Look for any synchronous entries.

The problem is a couple of months old. Everything was working just fine before that, and I had this PC for about 2 years or so.

I went to google and found a useful tool in this page for calculating your PC Power Supply Wattage (http://www.jscustompcs.com/power_supply/). According to them its about 250 Watts. My PSU is a 300 W Unit, so that should be Ok.

The Xp install was clean. I formated the disk before installing it.

I tried using only the 512MB stick in the first slot, but that didn't solve it.

I also tried running the chkdsk, but that didn't solve the problem either. So that leaves RAM and Hard drive out of the equation.

I forgot to add one thing that could be important: EVERY time xp loads (wether it crashes or not), after the xp logo appears the monitor turns off and on again for no reason before the desktop shows. It could be a clue as to what the problem is.

My best guest right now is that it's a video card issue (?).

Any ideas? (It would be a lot of help to solve the problem because my mid terms are getting close and i would really need my Pc in it´s best shape for that time...) THANKS!

If it is a Graphics Card issue there is not much to do but try another card.

My new PCI_E card also cuts out the monitor for a second between the XP logo and the desktop.

I don't know why it does this but it is apparently not a problem on my system.

Even though your Power Supply is adequate and basically working this is often the cause of behaviors such as you mention.

You could try a RAM test: http://www.memtest86.com/

Also you could run a diagnostic on your hard drive depending on the manufacturer:

Do you know the manufacturer of your hard drive?

You will find the major brand diagnostics listed here:

http://www.tacktech.com/display.cfm?ttid=287

With a few others listed here:

http://www.ultimatebootcd.com/