Built this system in 03

Asus A7N8X nForce2 Athlon(XP)/Duron SktA DDR ATX Motherboard
AMD Athlon XP 2500+ 1.83GHz 333FSB 512KB Processor
Kingston KVR400X64C25/512 512MB DDR400 PC3200 CL2.5
Western Digital Caviar Special Edition WD800JB 80GB ATA/100 7200RPM Hard Drive w/8MB Buffer
Asus CRW-4824A 48x24x52 EIDE CD-RW Drive w/Flextralink
eVGA NV52 GeForce2 MX-400 AGP 4X 64MB SDR Video Card
Sony FD MPF920Z161/131 1.44MB 3.5in FDD
Power Supply is Antec 350W.
OS is WIN XP Home

Only problem with the build was that during initial POST, motherboard did not recognize the processor. Went to website, got updated BIOS file, and flashed MB, everything great, with one exception, every time I went into CMOS and changed anything, had to reflash the BIOS for the MB to see the correct processor chip, EVERY TIME.

Since then, have added an additional HD, additional DVD drive, 2 gigs of memory, and changed the graphics card. Bout a month ago, machine started to shut down cold during processor intense apps (SpyBot, Sonar 3 recording software, etc), Opened case, and cleaned heat sink, fans, etc, made sure all cards were seated correctly, and added a additional 180mm front case fan. Now here is the problem, after machine has been flashed to see the AMD 2500+ chip, Windows may run for a while, but then the blue screen of death appears indicating a hardware error "MACHINE CHECK EXCEPTION" I think is the message. Reboot, machine sees AMD1100 chip, flash the BIOS, get the blue screen of death. I am now running the system and Windows without flashing the BIOS, and machine seems to be running fine. (Windows sees AMD 1100 chip, not the correct AMD2500+ chip). Never overclocked it, just ran standard setting in CMOS

When you opened the case to clean the heatsink did you remove the heatsink to clean it? If not, I would reseat the heatsink to be sure.

Wow, I had a custom built very close to that!
Asus a7v333
AMD ATHLON XP - 1.667 GHZ
512 MHZ Ram
Western Digital 80 GB and Western Digital 40 GB.
Asus CRW-4824A 48x24x52 EIDE CD-RW Drive w/Flextralink
eVGA NV52 GeForce2 MX-200 AGP 4X 32MB SDR Video Card
1.44MB 3.5in FDD
Power Supply is Antec 350W.
OS is WIN XP Home

But I had no problems with it. Even built it in the same year. And it still works.

Your problem is most likely hardware related. so by removing the non essential parts and starting up,replace them one at a time you can easily find what part is causing it.special attention to your newly installed HD and DVD. format your new HD if you not booting from it. if you do boot from it run chkdsk

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