failed install vista windows xp sempron 64 system

I have a long time problem that I cannot seem to resolve. I updated an older computer to the Sempron 64 (full system details below). I attempted to install WinXP Pro and while the install would complete, the computer would not boot in to windows. In fact, just as it is about to boot in to windows, it automatically reboots and it will continue doing this over and over and over. I can get in to safe made but can't figure out the trouble.
So I tried installing WinX64 beta. That worked fine and I assumed the issue was the 64 bit processor (even though everyone in the world can install a 32 bit OS on a 64 bit CPU.
Now that Vista beta is out and my X64 trial has expired, I decided to give it a shot. So I downloaded and burned the iso for Vista 64 bit, updated a hard drive and installed new speedy memory and did a clean install. Wouldn't you know it, as it is copying files I get a blue screen. (I should have taken down the error, but I did not). This is my husband's computer and he needed it, so I went back to X64 which installed and ran successfully. HOWEVER, after it installed, I ran an Update and 31 critical files were installed. After installation, the system rebooted but WOULD NOT load back in to windows again! Instead it started the constant rebooting problem I listed above.
I was able to get in to safe mode and examined my hardware. All the drivers were installed and the correct hardware was listed in the device manager. So I rebooted and tried to load the last good configuration of windows and that failed as well! My solution was to wipe everything and reinstall X64 and just shut down automatic updates.
As you can see, I am at a total loss here. Obviously, we cannot stay with X64 as it is only a trial. We are willing to go with Vista as we should (we hope) be able to upgrade it to a retail version without losing settings. Ideally, WinXP would be perfect for now. Has anyone been having problems like this? Here is my system:
Gigabyte GA-K8U Motherboard AMD Sempron64 2600 CPU Centon 1 GB 3200 DDR400 RAM (was previously tested with Samsung 2100 DDR266 memory and failed) nVidia GE Force 5200 AGP video card Western Digital 160GB SATA drive (was tested with Maxtor 40GB IDE and failed) ALC850 CODEC Sound (on board) Realtek (RTL8100C) 10/100 Network Adapter (on board) BENQ DVD drive (was also tested with Plextor 12/10/32 Plexwriter and failed)
Any ideas? denise@hvc.rr.com