PC failing to boot/acting odd

[Nizbot]

1. Overview of the problem
   PC boots up normally to the BIOS screen, but then get's to the vista load screen and starts acting funky/really slow. It then hangs, and reboots.
   
   
2. Full description of the problem and symptoms
   As vista is loading, the system becomes really choppy and slow. On the vista load screen, I see a few colored dots in a few places (possible vid card failure?). This is when it get's somewhat random. Sometimes, I can get to the login screen, albeit very sluggishly, and it'll slowly boot into windows. If I leave it for a few minutes it'll either a.) reboot b.) show bsod and hang on that screen c.)show bsod and restart. Other times when I start up, it won't even get to the vista load screen, and just go into a reboot loop after the bios screen loads.
   
   **I will post the BSOD error messages when I get home this evening.
   
   
3. Did it work normally at one time, or has the problem always existed?
   Yes. It has been working normally for about 2 years now with no major problems at all.
   
   
4. Is the problem consistent and repeatable, or entirely random, or semi-random?
   It seems to be somewhat random as I have mentioned in the full description above, although those three things happen consistently.
   
   
   
5. I already tried these steps:
   On startup, after it has crashed/rebooted, it will give me "Windows failed to start," and give me the boot into Safe Mode options. Tried booting into safe mode, but after loading those files, reboots again.
   
   Tried inserting the Vista dvd, and tried the repair option. Would get to where it shows the vista partition, acting really slow again, and hangs as it's trying to repair...reboots again.
   
   I have (3) 1g sticks of ram. Tried removing 2, seemed to work the first time. Tried switching that one out with one of the others, same reboot loop. Tried the third stick, same reboot loop. Tried placing the first one back in and it tried to reboot, acting really slowly again, then rebooted. I thought that was kinda of odd considering it seemed to work that first time.
   
   The memory is the only hardware I have tried testing so far. I will be testing more tonight.
   
   
6. My software:
   • Windows Vista 32; SP1
   • Avira AntiVir Personal
   • Windows Firewall
   
   
   
7. My hardware
   • ASUS P5LD2 Deluxe
   • Intel Pentium 4 651 Cedarmill 3.4ghz (not overclocked)
   • BFG GeForce 7900 GT PCI Express 256MB
   • Corsair 3x1GB DDR2-675 xms2-5400
   • Antec TruePower 650

 

[Nizbot]

Any suggestions what might be causing this?

 

[MagnusTheBrewer]

Possible bad ram, GPU, HDD, PSU or, MB. Usually in that order. Eliminate and consolidate. Unfortunately, about all you can do is start switching out components and testing.