Diagnostic Test boot loop

I've searched the web for days trying to figure something out. Found one 4-year-old post that was similar in the Dell desktop forums that suggested MBR problem, but no solution. Posting here as a last resort.

A few days ago, Vista would not boot on my Dell Studio 1535 laptop. Ran startup repair - no help. No system restore points could be found (even though I had set some in the past). Appeared to be booting to the recovery partition, chkdsk didn't help, bootrec could not find any Windows installs. I did all I knew how to do. Did some internet research tried some things, no solutions.

Somewhere as a result of one of the attempts (not sure which one), the machine is now in what seems to be a boot loop and will only boot to Dell's stand alone memory test/diagnostics menu. (It's a window with "Choose an Option" in the header, and three choices: Test Memory, Test System, Exit). Test Memory, runs the memory test (it passes). Test System, runs the diagnostic test (it passes). Exit, exits the diagnostic test menu and reboots the system which goes back to the diagnostic test menu.

F8 does nothing. F12 takes me to BIOS, the boot order is USB, CD, hard drive, but I can't boot from USB or disc, it keeps going back to the test menu. I cannot bypass it.

Any ideas? I'm helpless and clueless.  Thanks.