Windows 7 freezing, Out of ideas to fix

I have a Acer Aspire AX1700 that came with Vista and upgraded to Windows 7 Home Premium via digital download from Digital River. My computer has been working fine for the past 2 years. Recently my computer has been freezing and I have to do a hard reset. It will boot up and work fine for about 15 min or so before freezing up and having to do a hard reset. Even in safe mode it works for about 15 min before freezing up. I took it to a computer shop and the guy said my graphics card is bad so I replace it with a Radeon 5450. Still the same results. I took it back to him and he tried running without the Radeon card and it still froze. He said the hardware is fine to reinstall Windows. I don't have a recovery disk so I tried going to Geek Squad for a second opnion. He said it passed all the hardware test so it must be a os problem, to reinstall Windows. I tried doing the alt f10 to get to the recovery partion but it only goes to the device boot manager which I push enter it just boots up Windows. Sorry I made this post so long.

I will try your suggestions when I get home. One thing I forgot to add, I did a system restore to a earlier date and still the same thing. Thanks for the quick reply.

I went through about a month of my screen freezing, having to hard-boot, sometimes it would come up...some times it would come up with black screens (I have two monitors). It was a freakin' nightmare. Fortunately, I'm still under warranty. I got HP on the line & (long story short) it appears I allowed Windows to do a driver update to my GeForce 315 when it ended up hosing it up royally. On the 2nd pass to let Windows update, it backed out after most of the installation was done, said it couldn't continue but that it left me with drivers capable of running a VGA. ...Yeah, a mess that changed all icons and fonts to look like I was in Safe Mode and it didn't even recognize the 2nd monitor. HP helped me ascertain my drivers had actually been (first corrupted and then) REMOVED by Windows, get to the Nvidia site and (re)download the proper drivers. So far, no locking or freezing or blanking out ...and it boots up like it should. THANKS to HP for helping me unscrew a problem that belonged to Microsoft. SO, if you've religiously done your updates, check to see if you can rollback your vid card's changes past when you first started experiencing this. If you keep updating any new installation through Microsoft, you are continuing to download the new incompatible drivers they're giving you, IMHO.