Computer shuts off and restarts after POST

[Aaron Reinhard]

I have a pair of Dell Optiplex 745 (SFF) computers that I purchased off-lease.
They worked for a while hassle-free.
Lately, I've restarted both of them and get the following problem:
I press the power button and everything comes to life as usual. I get past POST and the Windows loading screen comes up. About 5 seconds into that screen, the computer cuts off all power. After about 3 seconds, it automatically comes back on and reboots.
They will repeat this cycle over and over until I manually hold the power button to cut the power completely.

Since I spend under $100 on these, I want to get opinions before I start putting more cash into parts. My first hunch was the power supply. However, I want to be sure before spending $30+ on new ones.

 

[johnb35]

I doubt its its the power supply. First look at overheating possibly. Another possibility is that windows boot loader is just corrupted, sort of like the one i'm working on currently. Hard drive tested fine. I scanned the drive attached to my system, no malware detected, no rootkits. So i can only assume boot loader or something else is corrupt. I may try repairing boot loader first before backing up data and doing a fresh install.

 

[Agent Smith]

Try booting the computer in safe mode. Press F8 repeatedly as the computer starts. If it boots you have a driver problem. If not it could be a bootloader issue. To fix that you will need to boot the OS disk. Press F12 repeatedly and chose to boot the CD drive with the OS disk. Select repair computer.