PSU doesn't necessarily have to 'closely' match the orginial. Just be sure it has suffeciant wattage. And has at least the minimum number of connection your configuration needs. For example, PCI x16...

Make sure you have good case cooling as well. Cool air in from the front and hot air out the back. I am also willing to bet that your 400W PSU is not sufficient for your recently installed upgrades.

RAM is def your problem. As compdoc mentioned above. The Maximum supported memory for this system is 512MB. You need two 256 MB Memory Modules for a Dell OptiPlex GX150 Series System.
Dell Part#...

From my experience Windows Defender isn't the greatest software for cleaning an infected PC. It does an alright job keeping the PC clean, but not so good at removal. Can you download, install,...

I was getting this error with a similar Dell. Ultimately I pulled the CMOS battery to reset the bios, put in a new PSU and a new CPU fan and heat sink. Been fine ever since. A faulty power supply and...

Have you tried LogMeIn? LogMeIn Rescue to be precise. No installations needed. The user goes to logmein123.com and you do the rest.

Run SCANPST.exe
This seemed to work for me (at least for now). The scanpst repair tool is installed by default
<system root>\program files\common files\system\msmapi\1033

Thanks for the response. As soons as I find something else that might help, I'll keep you posted. Please do the same.

Anyone ever solve this?

Have you been able to find a fix? Or even a cause? I have the same exact issue. I even tried to uninstall and reinstall the office suite, but the problem came right back. Don't know if this will help...