Hello and welcome to the windows7forums.
Right-click on the Computer icon on the desktop, choose properties, click the Advanced system settings, then the Setting button under Startup and Recovery and uncheck the box to Automatically restart.
Truthfully you just purchased this system, Sony should provide free technical support if you can understand the person on the other end of the line
I took it to Sony and they reinstalled everything - it still crashed!
Was it working OK after Sony re-installed. You didn't leave the store with it crashing, did you?
Try going to Start > Run, and type msconfig and click the Startup tab. Uncheck a;; entries except for your anti-virus, spyware detector, and personal firewall if you have one.
BTW, what anti-virus package is installed on your computer?
I don't think it is a Microsoft problem. Have them replace the unit if you can. If they are selling them with Win 7 pre-installed, they must have tested them. Did someone local do the pre-install and do you know if they did that by upgrading over a Vista install?
If nothing else, have them put a new hard drive in it and install fresh.