This started a few days ago after surfing with Opera 36 for a short while (I ordinarily use Firefox). Fired up my PC the next day and found no taskbar or icons on my desktop, just a cursor. Powered down the box, waited a minute, then started the machine again and XP loaded normally. Spooked, I updated and ran full scans with Bitdefender AV and MBAM. They didn't detect anything. Rebooted and the desktop was blank again. Discovered Task Manager worked and used it to close Windows.
I learned that clicking on my user icon as soon as it appeared during startup would load XP normally, but if I waited too long it wouldn't. Did a little research and followed Dell's recommendation to boot into Safe Mode and use System Restore to pick a earlier restore point. That seemed to work. Although my EMET icon was no longer in the tray and Bitdefender needed a repair install, the machine performed normally for a few hours. Opened EMET 5.0 and noticed it was consuming resources so I disabled it. To be on the safe side I downloaded, installed, and ran MS Safety Scanner; again nothing detected.
Still a little spooked, I downloaded the latest free standalone version of MBAE Beta. With EMET disabled and potential conflicts addressed, MBAE installed and ran fine. Didn't experience any issues until I powered up the box again a little later. My desktop loaded slowly and MBAE didn't launch. A window popped up informing me that MBAE was taking too long to load and advising a restart. I did so and MBAE again failed to load. Rebooted one more time and my blank desktop returned. Afterward it didn't matter how fast I clicked my user icon--got an empty desktop with every restart.
Went to Safe Mode. Updated and ran Kaspersky TDSS Killer which found nothing. Then used Panda's online malware scan; this scanner identified a hijacker and killed it. Still had a blank desktop. Thinking any malware was gone, I launched Task Manager on the empty screen and added explorer.exe to the Processes as a new task (Run...). Explorer.exe would be visible in the Processes list for a few seconds and then disappear. Returned to Safe Mode and System Restore. Tried 3 restore points and encountered 3 Restore failures.
I'm out of ideas. Just noticed that another poster here is having a similar issue with Win 8.1, so it looks like I'm not alone...