Question : Problem: PC reboots without warning after CPU upgrade

Thanks for any help, and here's the patient: I have a Pentium 4 1.6GHZ (northwood) in an Abit BG7 Motherboard with 512MB RAM. A few days ago, I upgraded the CPU to an Intel P4 2.8GHZ, 533MHZ, 512Kb cache (northwood), added 512MB of more RAM (1GB total), and added also another hard disk (Maxtor DiamondPlus 9 200GB 7200RPM). Both of my hard disks are Maxtor at 7200RPM. I run Windows XP Pro with all updates, SP2, etc. I changed the default CPU fan that came with the 1.6GHZ CPU to a Zalman fan that works in silent mode, and compatible with the CPU.

It happened that the silent mode was not enough to dissipate the heat from the CPU and I got warnings from the motherboard on the CPU's temperature, so I had to set it in normal mode. Now I can monitor the CPU and all hardware and temp. wise is fine. However, since I made the upgrade, WinXP would reboot at random while I'm in the middle of a task in Word, Frontpage, Photoshop, or any other application. I blame the CPU, maybe it is not correctly set up on the BIOS and causes application crashes that forces a reboot, but the curious thing is that I don't perceive a crash. It simply reboots out of the blue...

Oh, before I forget:

-Reboots occur when 3 or more programs are opened (Frontpage, PhotoShop, Internet Explorer, and Macromedia Flash for example). However, CPU usage never gets higher than 30% and RAM available is usually greater than 380MB.
- The RAM installed are all Kingston DDR266 PC2100 CL2.5, which is totally supported by the motherboard. Also, the tech specifications of the motherboard say it supports up to a 2.8GHZ CPU, 533MHZ FSB.

Any ideas?

Answer : Problem: PC reboots without warning after CPU upgrade

you can test the memory with memtest86+ from :    http://www.memtest.org/
you can set the FSb at a lower speed, to check if the cpu is the cause
Since you added  adrive, the power supply can be the cause too; calculate your needs at :

http://www.jscustompcs.com/power_supply/